We present a series of 43 infants with neonatal herpes simplex virus infection treated at the Children's Hospital, University of Helsinki, during a 16-year period from 1970 to 1985. Twelve mothers (28%) had a history of genital herpes during pregnancy, and two had had labial herpes infection. Eight infants (19%) were delivered by cesarean section. In 14 (33%) infants symptoms appeared within 24 hours and in 26 (61%) they appeared within 7 days. The presenting symptoms were neurologic in 79%, cutaneous in 30%, respiratory in 19%, cyanosis/pallor/grayish skin in 16%, irritability in 12% and fever in 7%. Herpes simplex virus was detected most early and frequently in pharyngeal swabs, in one-third on Postnatal Days 2 to 5. Cerebrospinal fluid contained an increased amount of protein and/or pleocytosis in 72%. Abnormal electroencephalographic background activity appeared in 56% and electrical paroxysms in 41%. Six infants (14%) died, 9 (21%) were damaged severely and 6 (14%) were moderately or mildly damaged. Poor prognosis was associated with acute maternal illness at delivery, prematurity, visceral involvement and/or electrical paroxysms in the electroencephalograms. This study underlines the occurrence of intrauterine transmission of herpes virus, infections with neurologic manifestations, early symptomatology and the need for prompt diagnosis, brain biopsy in selected patients and antiviral therapy in neonates with herpes virus infection.