In 1987, Washington became the first state to require that infection with Escherichia coli serotype O157:H7 be reported. In the first year of surveillance, 93 cases were reported, yielding an annual incidence of 2.1 cases per 100,000 population. The median age of case patients was 14 years (range, 11 months to 78 years), with the highest attack rate among children younger than 5 years (6.1 cases per 100,000 population per year). Bloody diarrhea was present in 95% of reported cases, 12% of patients developed either hemolytic-uremic syndrome or thrombotic thrombocytopenic purpura, and one patient died. Suspected secondary cases were seen in 5% of households. Fifty-six (60%) cases occurred during June through September, as did 73% of the cases of hemolytic-uremic syndrome or thrombotic thrombocytopenic purpura. Cases reported during the summer months were more likely than cases reported at other times of the year to be in children younger than 10 years. Medications, including antimicrobial medications, did not influence the duration of symptoms, nor did they appear to alter the risk of developing hemolytic-uremic syndrome or thrombotic thrombocytopenic purpura. This newly established surveillance system in Washington demonstrates that E coli O157:H7 is an important and common cause of bloody diarrhea in the United States.