We report on a case of cytomegalovirus (CMV) enterocolitis occurring in a 65-year-old patient after a first course of standard chemotherapy for a small cell lung carcinoma (SCLC). Colonic biopsies showed typical inclusion bodies, CMV IgM and IgG antibodies were found in the serum, and polymerase chain reaction for CMV-DNA was positive. Lymphopenia and diminished CD4 T-cell counts were observed. Diarrhoea ceased under ganciclovir treatment, the patient recovered, but died several months later of metastatic lung cancer. Significant immunosuppression leading to severe colitis by CMV infection or reactivation can occur after standard chemotherapy.