A 55-year-old patient with mant e cels underwent a cytotoxic chemotherapy (D.H.A.P. + Rituximab). During the medullar aplasia related to the third cycle, diarrhoea due to Clostridium difficile arised and relapsed 15 days later despite normal blood counts. This colitis was very severe with pluribacterial peritonitis, but resolved with intensive medical treatment. The incidence, the patient's risk factors, the iatrogenic and nosocomial characters of cl. difficile colitis are discussed.