Warren Buffett Warned About the Dot-Com Bubble. Now, the Stock Market Is Flashing the Same Warning Signal.

In 1999, Buffett warned that stock prices could begin declining.

Now, the metric he used to predict the downturn is flashing another warning signal.

After months of volatility and stagnating performance, the S&P 500 (SNPINDEX: ^GSPC) and Dow Jones Industrial Average (DJINDICES: ^DJI) both reached new record highs this week. While the tech-heavy Nasdaq Composite (NASDAQINDEX: ^IXIC) is still working through its recent pullback, it's surged by nearly 4% in the last five days alone, as of this writing.

However, all of this growth over the past few years has raised fresh concerns around valuations. Tech companies are spending massive amounts to build out data centers, with AI spending expected to top $5 trillion by 2030 , according to analysis from McKinsey & Company.

Missed Nvidia in 2009? This Rare Signal Is Flashing Again. In 2009, a "Double Down" signal flashed for a little-known chipmaker called Nvidia. For the first time in years, that same "Total Conviction" signal is flashing for a company 1/100th the size of Nvidia. Continue »

Some investors are drawing parallels to the dot-com bear market of the early 2000s, when hundreds of internet companies ballooned in value only to crash hard when the bubble popped.

Back then, Warren Buffett warned that stock prices were on the verge of a decline. Now, the stock market is flashing a warning sign yet again. Here's what investors need to know.

Former Berkshire Hathaway CEO Warren Buffett. Image source: The Motley Fool.