271 children born to HIV-infected mothers in 8 European centres are being followed up from birth in a multicentre, collaborative study. By June, 1988, 45% had been followed for over 1 year: 10 had developed AIDS or AIDS-related complex, all by the age of 9 months, of whom 5 had died. 22 other children had symptoms or signs suggestive of HIV infection; of these, 12 had immunological abnormalities, 9 of whom were infected. 5 children had problems not related to HIV, including 3 neonatal deaths. The other 234 children are immunologically normal and clinically well. The median age of antibody loss was 10.3 months, although 1 did not lose antibody until over 18 months. None lost antibody and then became and remained seropositive. Of 100 children followed for more than 15 months, 19 had persistent antibody, and 5 were antibody-negative but presumed to be infected because of virus isolation or antigen detection; these 5 children were clinically and immunologically normal. The estimated vertical transmission rate was 24%.