A retrospective analysis of all the cases of Clostridium difficile-associated diarrhea (CDAD) in hospitalized patients infected with HIV was performed over a 52-month period to assess the incidence, epidemiology, and risk factors of CDAD. A case of CDAD was defined as a patient with diarrhea and a positive stool cytotoxin B assay. Sixty-seven cases of CDAD were recorded in HIV-infected patients between January 1991 and April 1995. The annual incidence of CDAD ranged from 1.7 to 6.4 per 100 HIV-infected patients discharged from hospital. The 67 CDAD cases included 48 (72%) first episodes and 19 (28%) relapses. Serogroup C accounted for 69% of strains from initial episodes of CDAD. To identify risk factors for CDAD, 34 HIV-infected patients with a first episode were compared with 66 HIV-infected controls matched for the length of hospital stay. Three independent factors remained significantly associated with CDAD among HIV-infected patients: CD4+ cell counts <50/mm3 (OR = 5.2; 95% CI = 1.4-19.3; p = 0.01), clindamycin use (OR = 5.0; 95% CI = 1.3-18.3; p = 0.02) and penicillin use (OR = 4.6; 95% CI = 1.1-18.8; p = 0.03). C. difficile is a common enteric pathogen responsible for nosocomial diarrhea in HIV-infected patients. Clinicians should keep this pathogen in mind when searching for the cause of diarrhea in these patients, especially those who are severely immunocompromised or have received clindamycin or penicillin.