The May 20, 2003, announcement confirming diagnosis in a Canadian cow of mad cow disease caused price disturbances in livestock, grain, and stock markets. Price and time data are used to provide a clinical study on the timing, persistency, and rationality of those disturbances in different U.S. markets, showing the three types of uncertainty that C. Avery and P. Zemsky (1998) use to identify herd behavior and the resulting mispricing. Markets react at different times, showing an informational cascading pattern. Misperceptions cause futures contract and stock reactions that are unsupported by the facts. Livestock and grain futures markets reactions suggest that people would replace beef with pork. Biogenetic stocks show price disturbances for companies with no relation to screening or treatment for mad cow disease. The market reactions to the December 23, 2003, announcement of the first incidence of mad cow disease in the United States are examined to see whether the markets have learned from the May event.