To determine the magnitude and source of an outbreak of cryptosporidiosis among persons with human immunodeficiency virus (HIV) infection and to determine whether the outbreak extended into the immunocompetent population. Matched case-control study and environmental investigation. Clark County, Nevada. Adults with HIV infection (36 case-patients with laboratory-confirmed Cryptosporidium parvum infection and 107 controls), matched by physician or clinic and by CD4+ cell count category. Potential risk factors for infection, death rates, and data on water quality. Review of surveillance and microbiology records identified 3 cases of cryptosporidiosis in 1992 (the first year that cryptosporidiosis was reportable in Nevada), 23 cases in 1993, and 78 cases in the first quarter of 1994. Of the 78 laboratory-confirmed cases in the first quarter of 1994, 61 (78.2%) were in HIV-infected adults. Of these 61 adults, 32 (52.5%) had died by 30 June 1994; at least 20 of the 32 (62.5%) had cryptosporidiosis listed on their death certificates. In the case-control study, persons who drank any unboiled tap water were four times more likely than persons who drank only bottled water to have had cryptosporidiosis (odds ratio, 4.22 [95% Cl, 1.22 to 14.65]; P = 0.02). For persons with CD4+ cell counts less than 100 cells/mm3, the association between tap water and cryptosporidiosis was even stronger (odds ratio, 13.52 [Cl, 1.78 to 102.92]; P = 0.01). Additional data indicate that this outbreak also affected persons who were not infected with HIV. No elevated turbidity values or coliform counts and no Cryptosporidium oocysts were found in testing of source (Lake Mead) or finished (treated) water during the study period, but so-called presumptive oocysts were intermittently found after the investigation in samples of source water, filter backwash, and finished water. A cryptosporidiosis outbreak was associated with municipal drinking water, despite state-of-the-art water treatment and water quality better than that required by current federal standards. This outbreak highlights the importance of surveillance for cryptosporidiosis and the need for guidelines for the prevention of water-borne-Cryptosporidium infection among HIV-infected persons.