Speaking to reporters over dinner late last week, Altman drew a parallel between today’s AI frenzy and the 1990s dotcom bubble, when internet company valuations spiked dramatically before crashing.

“When bubbles happen, smart people get overexcited about a kernel of truth,” Altman said, in comments reported by The Verge. “If you look at most of the bubbles in history, like the tech bubble, there was a real thing. Tech was really important. The internet was a really big deal. People got overexcited.”

He noted some startup valuations for companies raising hundreds of millions of dollars with only a staff of three were “insane.”

“Are we in a phase where investors as a whole are overexcited about AI? My opinion is yes,” he said. “Is AI the most important thing to happen in a very long time? My opinion is also yes.”

Altman warned that some investors are likely to get “very burnt” as some of the hype unwinds, but maintained that the long-term value created by artificial intelligence will outweigh short-term losses. He also repeated the word “bubble” three times in 15 seconds, joking that the comments were likely to become a headline.

Wedbush’s Dan Ives, however, was undeterred by Altman’s slightly tepid tone. He told Fortune that the “AI revolution will fuel a tech bull market for the next two to three years at least.

“This is trillions being spent in the build-out of this fourth industrial revolution. There could be froth in certain areas of the private market for AI vendors, but ultimately, we do not see this as a bubble. This is a 1996 moment with a lot more room to go, not a 1999 moment in our view,” he said in an email.

Richard Saperstein, chief investment officer at Treasury Partners, also shrugged off concerns, noting that large-cap technology stocks remain the market’s driving force.

In a Monday note reported by Barron’s, he wrote that big tech companies “have led the market higher and will continue to dominate market performance,” citing expectations for continued earnings growth, strong reinvestment of cash flows, and the expansion of their global reach.

Saperstein advised investors to remain fully invested in U.S. equities, with a particular focus on large-cap technology names. He pointed to structural tailwinds, including deregulation, onshoring, and favorable treatment of capital expenditures, that he believes will support both corporate performance and broader economic growth in the years ahead.

No sign of a spending slowdown

Investors have had a reason to cheer in recent weeks, as major tech companies reported earnings that exceeded expectations. Microsoft, Alphabet, and Meta all posted strong growth and showed no signs of pulling back on AI.

The largest technology companies, including Microsoft, Amazon, Alphabet, and Meta, have all increased their capital expenditure forecasts to meet rising demand for artificial intelligence. Altman’s OpenAI is no different.

“You should expect OpenAI to spend trillions of dollars on data center construction in the not very distant future,” Altman said, in comments reported by The Verge. “And you should expect a bunch of economists wringing their hands, saying, ‘This is so crazy, it’s so reckless,’ and we’ll just be like, ‘You know what? Let us do our thing.’”

As AI spending soars, there has been simmering concern that investment in AI may be outpacing sustainable growth. Industry figures, including Alibaba cofounder Joe Tsai and Bridgewater Associates founder Ray Dalio, have all voiced concerns about the trend.

Earlier this year, Dalio warned that the current cycle on Wall Street appeared to be “very similar” to that seen before the dotcom bust in 1998 and 1999.

“There’s a major new technology that certainly will change the world and be successful. But some people are confusing that with the investments being successful,” Dalio told the Financial Times.

In a report last month, Apollo Global Management chief economist Torsten Slok went further, arguing that the current AI boom may surpass the internet bubble of the 1990s. He noted that the 10 largest companies in the S&P 500 are now more overvalued relative to fundamentals than during the peak of the dotcom era.