Clostridium difficile is now well-established as the main cause of antibiotic-associated diarrhoea or colitis. Nosocomial acquisition of C. difficile has been described among various immunocompromised patient populations. From January to December 1992, 19 cases of antibiotic-associated diarrhoea occurred among AIDS patients in the same ward of a hospital. They all harboured C. difficile in their stools. All the strains were retrospectively typed by the random amplified polymorphic DNA (RAPD) method using two different 10-mer oligonucleotides. Twenty-five C. difficile strains (isolated from 15 patients) yielded the same RAPD pattern, suggesting they were identical. The timing and location of infections indicated that transmission of this highly resistant spore-forming bacterium can occur not only by direct patient-patient contact but also at a distance in time and space. This is the first report of an outbreak of C. difficile-associated diarrhoea in AIDS patients. Epidemiological markers are needed to recognize and control common-source outbreaks; RAPD assay is a simple, rapid and effective way of distinguishing isolates of C. difficile.