Cyclocytidine (Cyclo-C) was evaluated as an antiviral chemotherapeutic agent in 10 children with herpes-group virus infections. The diagnoses of the patients were severe cytomegalovirus (CMV) syndrome with pneumonitis in acute leukemia (2), congenital or infantile CMV infections (3), herpes zoster encephalitis in acute leukemia (1), severe nosocomial varicella in compromised patients (2), varicella pneumonia (1) and acute encephalitis due to herpes simplex type-1 (1). Cyclo-C was effective in all the cases with acute infections, although it had no benefit on the course of chronic CMV infection with irreversible damages. However, complete or transient ceasing of viral shedding was obtained in all the 5 cases with CMV infections. From the present small open study, further evaluation of Cyclo-C as an antiviral agent especially in acute CMV infections is warranted.