The largest known waterborne outbreak of disease occurred in March-April 1993, resulting from an apparent breach of treatment in one of the Milwaukee, Wisconsin, water treatment plants, resulting in a widely disseminated exposure to Cryptosporidium in finished water. At the present time, human dose-response information is being developed for the infectivity and morbidity resulting from oral ingestion of Cryptosporidium. Prior work has shown that dose-response information can be used to arrive at risk estimates from microbial infection. It was the objective of this work to determine if the characteristics of the Milwaukee outbreak were consistent with the information from the human feeding trials. The particular approach used in this paper embodies the use of Monte Carlo uncertainty analysis, in which we attempt to calculate the most likely level of contamination present in Milwaukee during the period of exposure. This is then compared to actual measurements, and an inference is made.