Margin debt, which tends to spike alongside stock rallies and pullbacks. What is troubling is that much of the buying is being fueled by cheap debt.
“Margin debt at the end of January reached $451.3 billion (record levels), its fifth record month in a row. Margin debt returned and surpassed record levels set in July 2007 back in April when it topped $384.37 billion.” (New York Stock Exchange)
“Warnings signs: The indexes’ string of record highs; high levels of margin debt, or borrowings to finance stock buys; the slim number of prior bull markets that have lasted past this point; and valuations that are close to levels when stocks last peaked.” (MarketWatch)