Forty-five children with neonatal herpes simplex virus (HSV) infection, representing all known cases in the diagnostic records of four virological laboratories within a 15-year period, were followed up. Twelve children had died. Sixteen of the 33 survivors were healthy. Thirteen children had severe disabilities: all of them showed severe mental retardation; moreover, 11 were tetraplegic, one was hemiplegic with hydrocephalus and one had a pronounced behavioural abnormality. Four children had slight to moderate disabilities: one child was mildly mentally retarded and three, although mentally normal, had hemiplegia and delayed speech development, one of them having a learning disorder as well. Of these 17 neurologically impaired children 16 had ophthalmological abnormalities. EEG recordings were made in 29 patients in the neonatal period. They were markedly abnormal in 24 patients, 14 of whom had localized periodic complexes. An abnormal EEG was a bad prognostic sign. The neurological outcome was better in the HSV-1-infected children (10 cases) than in the HSV-2-infected ones (35 cases). Progressive or recurrent encephalitis was strongly suspected in two preterm children.