We describe the risk factors and microbiological findings of an outbreak of Clostridium difficile (CD)-related diarrhoea in the Medical Oncology Department of the Curie Institute. Screening for CD in stools was performed on 59 patients with diarrhoea and 146 patients without diarrhoea. Toxin secretion, serotyping (enzyme-linked immunosorbant assay) and genotyping (AP-polymerase chain reaction) were performed on 39 CD strains from 32 patients. The risk factors for toxigenic CD-positive diarrhoea were also investigated. Twenty-seven (46%) patients with diarrhoea and 12 (8%) patients without diarrhoea were CD-positive (P<0.001). Patients with diarrhoea were older (P=0.03). Chemotherapy was a risk factor for toxigenic CD-related diarrhoea (P=0.02) and antibiotic treatment was a risk factor only in those patients who were also receiving chemotherapy. Serotyping and genotyping showed that several strains were involved in this outbreak, with only two instances of patient-to-patient transmission, involving four and two patients.