In order to identify features associated with an increased risk of transmission of HIV from seropositive women to their offspring, 70 children of 58 HIV seropositive mothers were studied. Fifty-six children were followed prospectively from pregnancy; in 14 identified after the puerperium, obstetric notes were reviewed and stored serum was tested. Twelve infants of 10 mothers were HIV infected. Risk of transmission was increased in the first year after seroconversion; 5/9 infants born at this time were infected compared with 7/61 born subsequently (P < 0.001). Progression to stage IV in transmitters was more likely, occurring in the mothers of 9 infected children at a median of 3 years (range 0.5-6.5) and in mothers of 19 non-infected children at a median of 5 years (range 1-7) (P = 0.032). Maternal CD4+ counts < 400 x 10(6)/l were found in 7/12 transmitting and 7/49 non-transmitting pregnancies (P < 0.01). Differences in HIV antigenaemia did not reach significance. These factors may influence the counselling of mothers regarding their child's and their own prognosis.