Patients with active diarrhea caused by infection with Cryptosporidium parvum can potentially contaminate the environment, which could serve as a risk for transmission to other patients in a hospital setting. A retrospective cohort study was performed to quantify the risk of nosocomial roommate-to-roommate transmission of Cryptosporidium and to evaluate the need for isolation of Cryptosporidium-infected patients. Thirty-seven human immunodeficiency virus (HIV)-infected roommates of 21 index patients with Cryptosporidium were identified between 1994 and 1996. Each exposed roommate (median CD4 cell count, 27cells/mm(3)) was matched to an HIV-infected, unexposed roommate with a similar CD4 cell count (median, 24 cells/mm(3)) who was present in the hospital during the same month but was not a roommate of a patient with Cryptosporidium infection. No patients with Cryptosporidium were identified among the 37 exposed roommates, and 1 case was identified among the 37 unexposed roommates. The risk ratio for chronic diarrhea was 0.80 (95% confidence interval [CI], 0.23-2.75) and for death was 1.04 (95% CI, 0.75-1.44). These results suggest that isolation of adult patients with Cryptosporidium diarrhea is not necessary to prevent roommate-to-roommate transmission of Cryptosporidium.