Contamination of the public water supply in Milwaukee during March and April 1993 resulted in a massive outbreak of cryptosporidium infection. We investigated the clinical and epidemiological features of visitors to the Milwaukee area in whom cryptosporidiosis developed, and we conducted a telephone survey of Milwaukee County households to evaluate the risk of recurrent illness and secondary transmission. Cryptosporidium infection during this outbreak generally seemed more severe than cases described in previous reports of large case series. The risk of secondary transmission within a household was low (5%) when the index case involved an adult. The recurrence of watery diarrhea after apparent recovery was a frequent occurrence among visitors with laboratory-confirmed cryptosporidium infection (39%) and among visitors and Milwaukee County residents with clinical infection (21%). The interval between the initial recovery and the onset of recurrence was prolonged (> or = 5 days) in 6%-8% of persons. This pattern of recurrence and its impact on transmission and our understanding of the pathophysiological mechanisms of cryptosporidium infection merit further investigation.