To assess the total medical costs and productivity losses associated with the 1993 waterborne outbreak of cryptosporidiosis in Milwaukee, Wisconsin, including the average cost per person with mild, moderate, and severe illness, we conducted a retrospective cost-of-illness analysis using data from 11 hospitals in the greater Milwaukee area and epidemiologic data collected during the outbreak. The total cost of outbreak-associated illness was 96.2 million US dollars: 31.7 million US dollars in medical costs and 64.6 million US dollars in productivity losses. The average total costs for persons with mild, moderate, and severe illness were 116 US dollars, 47 US dollars, and 7,808 US dollars, respectively. The potentially high cost of waterborne disease outbreaks should be considered in economic decisions regarding the safety of public drinking water supplies.