A massive outbreak of watery diarrhea was caused by cryptosporidium oocysts that passed through the coagulation and filtration system of one of Milwaukee, Wisconsin's water treatment plants. At first numerous cases of gastroenteritis were observed and reported to public health offices. These had resulted in widespread absenteeism from schools and hospitals. In early April, laboratories isolated Cryptosporidium oocysts from the stool of several persons. At this point, none of the laboratories had detected an usual increase or pattern in isolations of cryptospordium. On April 7, public health began active surveillance for enteric pathogens. Laboratories were asked to report test results for all stool specimens submitted for bacterial and viral culture and for ova and parasite testing. All positive tests for cryptosporidium from March 1 to May 30 were also requested. Household surveys in the greater Milwaukee area showed that the risk for developing watery diarrhea was highest in the southern region of Milwaukee's water system. By applying the rate of watery diarrhea found among household survey participants, it was estimated that 419,000 persons had watery diarrhea during the survey period of March 1 through April 28. It was theorized that more than half the people who received residential drinking water predominantly from the southern water-treatment plant became ill, which was twice the rate of illness among people whose residential drinking water came mainly from the northern water treatment plant. The Milwaukee Water Works (MWW), which obtained water from Lake Michigan, supplied treated water to residences and businesses in the City of Milwaukee and nine surrounding municipalities in Milwaukee County. Either of two water treatment plants, one located in the northern part of the city, and the other in the southern part, could supply water to the entire district; however, when both plants were in operation, the southern plant predominantly served the southern portion of the district. Examination of the water plants' records revealed an increase in the turbidity of treated water from the southern plant, beginning approximately on March 21, with increases to unprecedented levels of turbidity between March 23 through April 5. These findings pointed to the water supply as the likely source of the diarrhea outbreak. Water from ice blocks produced between March 25 and April 9 from water made in southern Milwaukee was tested and found to contain Cryptosporidium oocysts. A boil water order was issued by April 7. The southern water treatment plant was temporarily closed on April 9.