Three outbreaks of waterborne disease have been attributed to Cryptosporidium--two linked to drinking water and a third to surface water-yet the risk of waterborne disease in unknown because many factors may contribute to transmission. Of 107 surface water samples collected in six western states, 77 were positive for the presence of Cryptosporidium oocysts. A high count was found in raw sewage (1,732 oocysts/L), whereas low counts were found in waters without waste discharges (0.04 oocysts/L). Cryptosporidium has also been detected in drinking water. Little information is available, however, on oocyst survival in the environment or during sewage and drinking water treatment processes. Further research is necessary to define the variables that will influence the possible presence of infectious oocysts in water. It has been suggested that the epidemiology and transmission of Cryptosporidium are similar to Giardia. Based on environmental occurrence, the risk of Cryptosporidium transmission by the water route may be equal to or greater than that of Giardia. Includes 50 references, tables, figure.