CryptoKitties is a game where you breed and trade cartoon cats, each one an NFT on the Ethereum blockchain. It launched in November 2017 and became the first blockchain game to reach a mainstream audience, so popular for a few weeks that it slowed down Ethereum for everybody. This article explains how the game works, why it mattered so much, and what happened to it after the hype.
Risk note: CryptoKitties and other NFTs are speculative collectibles. Most of them are worth a small fraction of what they sold for in 2017 and 2018, and some cannot be resold at all. Nothing here is financial advice.
CryptoKitties in short
| Made by | Axiom Zen, a Canadian studio, later spun off as Dapper Labs |
| Launched | 28 November 2017, after a test version at the ETHWaterloo hackathon on 19 October 2017 |
| Blockchain | Ethereum |
| What you own | Individual cats, each a non-fungible token with its own genes |
| Core loop | Buy cats, breed them, sell or keep the offspring |
Dates and company details come from the CryptoKitties article on Wikipedia, which cites the original press coverage.
How the game works
There is no battle system and no story. You hold cats in an Ethereum wallet, and every cat has a genome that decides how it looks. Wikipedia counts 12 visible traits, which the game calls “cattributes”, such as fur pattern or eye shape, and puts the number of possible cats at around 4 billion.
The interesting part is breeding. You pair two cats, one as the sire and one as the dame, and after a waiting period the dame gives birth to a kitten whose genes mix both parents, with the odd random mutation. Rare traits made a cat more valuable, so players spent a lot of time and ETH trying to breed specific combinations.
You could also offer your cat as a sire to other players for a fee. That turned popular bloodlines into a small business inside the game.
Generations and cooldowns
Every cat has a generation number. The first cats, Generation 0, were released by the game itself: one every 15 minutes for a year. Cats bred from them get higher generation numbers, and the higher the number, the longer a cat has to rest before it can breed again.
This rest period is the cooldown, and it gets longer each time a cat breeds. The official guide lists these starting speeds:
| Generation | Name | Starting cooldown |
|---|---|---|
| 0 to 1 | Fast | 1 minute |
| 2 to 5 | Swift | 2 to 5 minutes |
| 6 to 9 | Snappy | 10 to 30 minutes |
| 10 to 13 | Brisk | 1 to 2 hours |
| 14 to 17 | Plodding | 4 to 8 hours |
| 18 to 21 | Slow | 16 to 24 hours |
| 22 to 25 | Sluggish | 2 to 4 days |
| 26 and up | Catatonic | 1 week |
Source: the CryptoKitties guide on cooldown speed. The guide’s own example is a Generation 11 cat that starts at one hour and reaches eight hours after three litters. In practice this design made low-generation cats scarce and expensive, because they were the only ones that could breed quickly over and over.
For this table we used the official guide rather than fan wikis, some of which give a different time for generations 16 and 17 (five hours instead of eight). If you ever check a cat’s stats, trust the guide.
What breeding cost
Every action was an Ethereum transaction, so you paid gas on top of any price. Breeding also carried a birthing fee in ETH. In December 2017 the team raised that fee from 0.002 to 0.015 ETH to keep up with demand. Small numbers on paper, but they added up fast for players breeding dozens of cats a day while gas prices were spiking.
Why CryptoKitties broke Ethereum
In early December 2017 the game was responsible for roughly a quarter of all traffic on Ethereum, according to Wikipedia. Transactions queued for hours and fees rose for every app on the network. CoinDesk reported at the time that the kitten traffic pushed at least one startup to delay its token sale.
Prices followed the attention. A cat called Genesis sold for 246.9255 ETH, about $117,712 at the time, in December 2017, and another cat changed hands for $140,000 in May 2018. By October 2018 players had bred 1 million cats across 3.2 million transactions.
The congestion was the real lesson. One collectible game was enough to clog a network that thousands of other projects relied on, and that pushed the whole sector to look for cheaper, faster chains for games. You can trace a straight line from those weeks to the gaming networks we cover in our explainer on web3 gaming.
The legacy: ERC-721, Dapper Labs and Flow
CryptoKitties left three things behind that outlasted the game itself.
The first is a standard. Dieter Shirley of Axiom Zen is one of the four authors of ERC-721, the Ethereum standard that most NFTs still use. The document lists CryptoKitties as compatible with an earlier version of the standard and points to its contract as prior art several times.
The second is a company. Axiom Zen spun the game off as Dapper Labs in March 2018, which raised $12 million that month in a round led by Union Square Ventures and Andreessen Horowitz, and another $15 million in November 2018 led by Venrock.
The third is a blockchain. Having watched their own game choke Ethereum, Dapper Labs built Flow, a chain designed for collectibles and games. Decrypt’s explainer on Flow tells that story, and Flow went on to host NBA Top Shot, a far bigger commercial hit than the cats ever were. In March 2021 the team announced plans to bring CryptoKitties to Flow as well.
What happened to CryptoKitties
The game faded quickly once the 2017 rush ended. Wikipedia notes that by 2022 it was seeing fewer than 100 sales a day, worth less than $10,000 in total. Breeding for profit only worked while new buyers kept arriving, and when they stopped, prices fell for everyone holding cats.
That pattern repeated a few years later with play-to-earn games such as Axie Infinity, which we look at in our guide to the Axie marketplace. A game economy that depends on newcomers buying in tends to shrink as soon as growth slows.
The project has not vanished. When we visited the official CryptoKitties guide on 1 October 2026 it was still online, with its sections on breeding and selling. Activity, though, is a long way from 2017, and you should treat any cat you buy today as a collectible with an uncertain resale market.
Should you still play it?
As a piece of history, yes, it is worth a look. Browsing the cats and reading how breeding works shows you in ten minutes why NFTs caught on and why they became so hard to price.
As a way to make money, no. Gas on Ethereum still costs real money for every breed and sale, the buyer pool is small, and old cats are illiquid. If what you want is a blockchain game that is actually fun to play, you will get more out of something like Gods Unchained, which is free to start, or one of the titles in our list of the best NFT games.