Infections were considered to be etiological factors in 29 patients (10%) with infantile spasms; congenital CMV (n = 5), congenital or acquired CMV (n = 1), acquired CMV (n = 5), congenital rubella (n = 2), herpes simplex virus (n = 5), enterovirus (n = 1), adenovirus (n = 1), viral encephalitis of unknown agent (n = 3), meningococcus (n = 4), pneumococcus (n = 1) and pertussis (n = 1). The children with congenital infections had long-lasting tremor and convulsions from birth. Early EEG pattern was characteristic for children with herpes encephalitis but not for other patients. Infantile spasms appeared only some weeks after viral encephalitis. One patient with enterovirus and another with probable adenovirus infection had necrotic changes in their brain CT resembling those of herpes encephalitis. The response to ACTH was poor (38%) compared to the whole series (60%). The long-term outcome was also poor compared to the whole series; mental retardation in 90%, convulsions in 62%, abnormal EEG in 89%. Four children died during the follow-up of 7 years. Autopsy showed disseminated CMV infection in one patient and chronic CMV infection in another. The outcome of children with infectious etiology appears to be particularly poor. Thus, the prevention and specific diagnosis and treatment are important. Steroid therapy should be avoided in children with a history of herpes virus encephalitis (CMV, herpes simplex) in the past.