This paper investigates the possible formation of informational cascades by traders in a random sample of 8,000 NYSE stock-days experiencing extreme price changes during 1998–2001. Our cascade measure is designed to detect informational cascades in high-frequency stock market prices. First, we find evidence of cascades on approximately 12% of the days when the NYSE experiences, on average, large price increases. This percentage increases to about 20% on days experiencing large price decreases, on average. Second, we find evidence that the interarrival times of trades in those stocks exhibiting significant informational cascades are generated by a nonlinear stochastic process. Third, the evidence supporting cascades is largely confined to smaller stocks and to stocks followed by fewer security analysts. Last, the occurrence of cascades appears to correlate with the incorporation of fundamental information into security prices.