Clostridium difficile is considered the most common cause of nosocomial acquired diarrhoea, with frequencies differing widely from one institution to another. So far, it is a scarcely reported condition in Spain. In the present study 129 episodes of Clostridium difficile associated diarrhoea (CDAD) occurred in 120 patients in a 2,000-bed hospital in 1994 is reported. All cases were diagnosed by demonstrating cytotoxicity on cellular lines (MRC-5) from feces or from the strain isolated from a culture medium (CCFA). The overall incidence was 2.4 episodes every 1,000 admissions. Twenty-eight out of the 120 patients (23%) were HIV-positive patients, that is, an incidence of 30 episodes every 1,000 admissions. No significant differences were observed regarding the presentation and clinical course between HIV-positive and HIV-negative patients, with the exception of the antimicrobial agents used previously. Forty-two percent of patients had undergone surgery and 97% had received antimicrobials in the 8 weeks before the CDAD episode, with an average of 3.3 antibiotics per patient. Out of the 129 episodes, 72.8% were treated correctly. A total of 11.7% of patients responded exclusively to the discontinuation of the antimicrobials that were being administered. Eighty-three patients were treated with specific antibiotics, 59 with oral vancomycin, and 24 with metronidazole. Seventy-six patients (91.5%) responded to the initial therapy, 5 relapsed (6%), and 2 (2.5%) failed. The associated mortality rate was 0.7%. C. difficile can be a relevant cause of nosocomial diarrhoea in our setting, particularly in HIV-positive patients, but also in other patients. Its early diagnosis and appropriate therapy can contribute to decrease a relevant cause of morbidity in inpatients.