The epidemiology of Clostridium difficile-associated diarrhea (CDAD) is not well defined in nonepidemic situations because precise biotyping techniques have only recently become available. Arbitrarily primed polymerase chain reaction (AP-PCR) was used to determine strain identity of C. difficile isolates recovered on our oncology ward, at an incidence rate of 0.84%. Twenty-one strains of C. difficile, which were grouped into 18 different AP-PCR types, were isolated from patients' specimens. Forty-two C. difficile isolates recovered from the environment (33 toxigenic and 9 nontoxigenic) represented 9 different AP-PCR types. The most commonly found type, a toxigenic strain accounting for 29% of the environmental isolates, was widespread throughout the ward. None of the environmental types were found among the isolates from patients. Three patients' isolates were of the same AP-PCR type, and two of these patients had occupied neighboring rooms at the same time. The diversity of C. difficile isotypes suggests that endemic nosocomial CDAD is not necessarily clonally spread.