This $1 Trillion Wall Street Warning Is Flashing Red. Here's What History Says Happens Next.

Key Points

• Margin debt is hitting record highs and growing at a pace not seen since 1999 and 2007.

• High levels of margin in the market can worsen natural downturns.

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Imagine it's 1999, and markets are hitting high after high. The dot-com boom is in full swing, and investors are euphoric seeing the value of their portfolios soar. For many, the advent of the internet meant that "it was different this time" -- technology stocks seemed like they would never stop their incredible march upward. I probably don't have to tell you what happened next.​ ​

While it might seem clear in hindsight -- maybe even obvious -- it's never the case when you're living it. It's easy to get caught up in the moment and miss the signs. And frankly, that may not always be the worst thing; bulls tend to outperform bears in the long run.

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But with the S&P 500 (SNPINDEX: ^GSPC) hitting new highs, many investors would love to know when the next crash is coming -- I sure would -- so it's useful to look for parallels between now and major market downturns of the past. Were there specific warning signs in 1999 and 2007 that a savvy investor could have seen before the crashes of 2000 and 2008?

Margin debt hits $1 trillion for the first time ever

One potential warning sign is the money traders borrow to invest in stocks, known as margin debt. This metric recently hit an all-time high, topping $1 trillion for the first time in June and rising again in July. But then again, the stock market is hitting new highs itself, so margin debt isn't setting records relative to the total value of the S&P 500.

​What is truly concerning is not how much debt there is in the market but how fast it's growing. Between May and June, leveraged positions grew 18%, the fifth-largest increase on record. The only two-month periods with higher growth rates all came in -- you guessed it -- either 1999 or 2007.

Why margin debt matters

Investors should care about margin debt for two reasons. First, high levels can accelerate a downturn. Traders who use margin cannot let the value of their portfolio fall below a minimum level in relation to the amount they borrowed in the first place. If stocks keep going up, that's not a problem.​

Image source: Getty Images.

If stock prices fall, however, and their portfolio dips below that minimum value, they face a "margin call" and must either add cash to raise the portfolio value or sell the stocks they bought with margin. Many don't have the cash on hand to pursue the first option and must sell. This can cause a runaway downward spiral as traders liquidate part of their portfolios to "cover" margin calls, which in turn lower stock prices further, leading to more liquidations, additional sales, and so on.

The second reason it matters is that it is a clear barometer of investor sentiment. A rapid increase, such as the one that recently occurred, suggests that investors are chasing growth. They appear confident that stocks will only go higher and are willing to take on an unusual amount of risk to capitalize on that. And while confidence supports markets, overconfidence fuels bubbles.

Here's what history says happens next

This rapid rise in margin was exactly the kind of warning sign investors could have looked for in both 1999 and 2007. History would seem to say that what happens next is a crash. However, it's critical for investors to keep three things in mind.

First, this is a single indicator in what is an incredibly complex market. If you look hard enough, you can probably find numbers that parallel just about any year. It's more than possible that a crash does not follow in the near term, and the bull run continues.

Second, there are numerous ways in which the market of 2025 differs from those of 2007 and 1999. The companies at the top of the food chain, like Nvidia and Microsoft, are mature companies with robust earnings and valuations that are significantly lower than those of a company like Cisco in 1999. In 2007, the risks posed by a housing market collapse went well beyond the market and equity prices. They were systemwide risks to the very foundation of the real economy.

And finally, even if this is a bubble, timing markets is almost never a winning strategy. Bubbles can keep going for quite some time. So, the lesson history has to offer here is that you should always look to invest in a diverse portfolio of solid companies for the long haul, rather than chasing the latest fad. This gives you the confidence and peace of mind to weather the natural ups and downs of the market -- even the big ones.​

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Johnny Rice has no position in any of the stocks mentioned. The Motley Fool has positions in and recommends Cisco Systems, Microsoft, and Nvidia. The Motley Fool recommends the following options: long January 2026 $395 calls on Microsoft and short January 2026 $405 calls on Microsoft. The Motley Fool has a disclosure policy.