The European Collaborative Study (ECS) on children born to human immunodeficiency virus (HIV) infected women is a multidisciplinary study of 19 centres with 1130 mother child pairs enrolled at the end of April 1992. The ECS has reported a rate of vertical transmission of 14.4%. Children born before 34 weeks gestation were more likely to be infected than were children who were breastfed. Transmission was high with vaginal deliveries in which episiotomy, scalp electodes, forceps or vacuum extractors were used but only in centres where these procedures were not routine. Transmission was not associated with maternal intravenous drug use, maternal age, race nor parity. There was no significant variation in transmission rates between centres. Although the relative importance of transmission at delivery is becoming apparent, prospective studies have shown similar rates of transmission in children delivered vaginally and by caesarian section (CS). There was some evidence of a protective effect of CS but the result was not statistically significant. Further research is needed to clarify the issue of transmission during labour and delivery. About one third of infected children develop acquired immune deficiency syndrome (AIDS) within the first year of life and 15% die of an HIV related death. The early onset of very rapidly progressive disease in a subset of children in the first 6 months of life needs further investigation. Although 90% of infected children show some manifestation of HIV infection before the age of 12 months, more children improve than deteriorate over the second and third year of life. The long term prognosis for infected children remains to be determined. Research is continuing on the clarification of the role of risk factors already identified, and the investigation of further risk factors for mother to child transmission of HIV infection.