"Somebody gave me some for my birthday," Gates laughs during an interview with Berkshire Hathaway CEO Warren Buffett and Vice Chairman Charlie Munger. "A few years later, I thought, 'Hey I'm going to sell that.'"

The Microsoft co-founder revealed Monday on CNBC's "Squawk Box" that he doesn't own any cryptocurrency, but held some briefly after it was given to him as a gift.

Bitcoin is "one of the crazier speculative things," Gates says. And, he's willing to bet against its success, adding, "I would short it if there was an easy way to do it."

Gates explained that one reason he has a negative forecast for cryptocurrency is because he sees the digital tokens as lacking intrinsic value.

"As an asset class, you're not producing anything and so you shouldn't expect it to go up," Gates says on "Squawk Box." "It's kind of a pure 'greater fool theory' type of investment."

Buffett explained that theory in an interview with Yahoo Finance like this: "You're just hoping the next guy pays more. And you only feel you'll find the next guy to pay more if he thinks he's going to find someone that's going to pay more."

In addition to bitcoin's value, Gates raised other concerns about digital currencies, which operate on a decentralized network without governance from a central authority, in an "Ask Me Anything" post on Reddit earlier this year. Namely, Gates took issue with the technology's anonymity.

"The government's ability to find money laundering and tax evasion and terrorist funding is a good thing," Gates wrote. "Right now, cryptocurrencies are used for buying fentanyl and other drugs, so it is a rare technology that has caused deaths in a fairly direct way."

Gates once had a brighter outlook on Bitcoin. In 2014, Gates said cryptocurrencies had benefits for transacting efficiently. "Bitcoin is better than currency in that you don't have to be physically in the same place and, of course, for large transactions, currency can get pretty inconvenient," he told Bloomberg that year.

But despite his current concerns with cryptocurrencies, Gates remains interested in the blockchain technology behind bitcoin, the digital ledger where transactions are confirmed and recorded.

"There's some really good technology in terms of sharing databases and verifying transactions that is talked about as blockchain, that is a good thing," Gates tells CNBC. In fact, the Bill & Melinda Gates Foundation gave a $100,000 grant to blockchain company Bitsoko in 2015. Last year, the foundation also partnered with Ripple, creator of the cryptocurrency XRP, to develop mobile payment technologies for the poor.

Gates isn't the only billionaire who's been gifted bitcoin. In February, Elon Musk tweeted that someone gave him a fraction of a bitcoin years ago, and its the only amount of the digital currency he owns.

"I literally own zero cryptocurrency, apart from .25 BTC that a friend sent me many years ago," Musk tweeted.

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