Five of six patients with the post-perfusion syndrome (PPS) following cardiopulmonary bypass had evidence of cytomegalovirus (CMV) infection. An additional patient who died in the immediate postoperative period had asymptomatic viruria preoperatively. One patient in whom postoperative lymphocytosis, hepatomegaly, and viruria developed had preoperative cytologic evidence of infection. In this study of 25 patients, the existence of latent infection prior to operation was suggested in this single patient. Nineteen other patients studied prospectively had no evidence of CMV infection or PPS.