To diagnose possible cytomegalovirus (CMV) infection in a 64-year-old man after renal transplantation, polymerase chain reaction (PCR), pp65 antigenaemia assay (pAA) and virus isolation in cell culture were routinely performed on a weekly basis. The PCR obtained virus DNA in peripheral blood lymphocytes for the first time in the fifth week. Two weeks later the patient complained of feeling unwell with abdominal pain and vomiting on eating. Two days later he developed a fever up to 38 degrees C and nocturnal sweats. Gastroscopy revealed marked antral gastritis which histologically showed typical cytomegalic "owl-eye" cells. The pAA was clearly positive and the cell culture started in the fifth week now showed a cytopathogenic effect. CMV gastritic having been diagnosed treatment consisted of 175 mg ganciclovir intravenously twice daily for 10 days. He became symptom-free after two days. The only side effects were thrombocytopenia down to 67,000/microliters and a rise in transaminase activities, changes which regressed later. To ensure early diagnosis and treatment of any CMV infection, specific virus diagnostic tests should be routinely undertaken after transplantation of organs from CMV-positive persons into CMV-negative patients.