Pseudomembranous colitis is frequently associated with antibiotics and more rarely with chemotherapeutic agents such as 5-fluorouracil. The objective of this study is to show that it is possible to confuse this infection with chemotherapy associated toxicity. We present a 54 year old woman who underwent surgery for colorectal cancer and in the first cycle of chemotherapy with 5-fluorouracil developed pseudomembranous colitis. We detected the toxin B of Clostridium difficile in stools and we began early antibiotic treatment. Thus, in patients with post chemotherapy neutropenia and diarrhoea that develop negatively, we have to rule out this infection.