At the Children's Hospital, University of Helsinki, 205 children with infantile spasms who were born between 1960 and 1976 were studied in a search for the factors responsible. In 11 children (5%) the infantile spasms were possibly associated with cytomegalovirus (CMV) infection. The number may actually have been considerably higher, as no systematic search was made for CMV, especially in the early years. In four of the 11 children, the infection was probably congenital, and was the most likely cause of the spasms. One of the remaining seven children had congenital toxoplasmosis and the simultaneous CMV infection was probably also congenital. The children with congenital CMV infection exhibited severe clinical symptoms in the neonatal period or in early infancy. Two frequent symptoms were persistent tremor and meningoencephalitis. Later, all five children were severely mentally retarded and had spastic tetraplegia and small heads; three of them had optic atrophy and were blind. In the other six children, the CMV infection was probably acquired, the clinical symptoms being less severe, and the spasms may have been caused by another factor. In two of the 11 cases, immunosuppressive therapy (ACTH treatment generally given for infantile spasms) seems to have caused a fulminant CMV infection. Two children with CMV infection still show signs of a slow virus infection in the central nervous system many years later.