From 1953 to 1963, 13 acute leukemia patients had cytomegalic inclusion disease at autopsy. This represents 39% of the total 394 autopsies on patients with acute leukemia. The salivary gland virus infection was the cause of death in 3 patients. These 13 patients with cytomegalic inclusion disease were compared with an age and sex matched control group. The clinical course of the patients in both groups were similar. There were no symptoms, physical signs or laboratory findings which suggested the presence of cytomegalic inclusion disease. The lungs were infected by salivary gland virus in all of the patients and the gastrointestinal tract in 11 of them. In one patient, the lung was the only site of cytomegalic inclusion cells. The disease was widely disseminated in 5 patients, in 3 of whom it caused death. In one patient, cytomegalic inclusion cells could be identified in 15 different sites. Eleven of the 13 patients with cytomegalic inclusion disease died in a 13 month period, suggesting that an epidemic had occurred. Eighty per cent of these 11 episodes occurred in children less than 10 years of age. Only 30% of children in this age group have significant titers of complement-fixing antibodies to salivary gland virus. These 2 facts support the concept that cytomegalic inclusion disease associated with acute leukemia arises as a primary infection.