This study was set up to estimate the incidence of neonatal herpes simplex virus (HSV) infection in the British Isles, and to document the outcome of neonatal infection. Paediatricians reported cases of neonatal HSV through the active reporting scheme of the British Paediatric Association Surveillance Unit. Over a 5 1/2 year period (1986-91) 76 infants with neonatal HSV infection were reported, an incidence of recognised infection in the British Isles of 1.65/100000 livebirths. Twenty-five infants had HSV-1 infection, 24 HSV-2 and in 27 virus type was unknown. Twenty-seven had disseminated infection, 23 herpes encephalitis and 26 localised infection. Nineteen infants (25%) died in the neonatal period, and a further 25 (33%) have subsequently died or have long-term sequelae. At least half of the infants had been discharged home before symptoms became apparent. For 21 women there was evidence of a maternal genital herpes infection at some time, but this was reported or diagnosed retrospectively after neonatal HSV was suspected in 19 cases, and antenatally in only two. Neonatal HSV is rare in the British Isles and routine antenatal screening for genital herpes infection during pregnancy is not justified. A high proportion of infected infants present with non-specific signs and symptoms and without mucocutaneous involvement; furthermore, there is rarely a history of maternal infection. As early diagnosis and prompt treatment is essential, there must be a high level of awareness of the serious nature of neonatal HSV infection.