We assessed the use of the polymerase chain reactions (PCR) with an arbitrary primer (AP-PCR) to investigate a major hospital outbreak of diarrhoea due to Clostridium difficile. A single pattern consisting of three bands of 240, 580 and 1100 bp was obtained from all isolates studied. AP-PCR is a simple, rapid technique which should find increased application in the rapid investigation of suspected outbreaks of many different bacterial species, particularly new pathogens or those for which no accepted typing scheme exists.