A nationwide, longitudinal study of infants born to human immunodeficiency virus-seropositive mothers has been under way in France since 1986. After 7 years of follow-up observations, we will update our assessment of the transmission rate in France and analyze, on a larger number of mother-infant pairs, the influence of maternal factors. Among the 848 pairs included in this analysis, the transmission rate was 20.2 +/- 2.7%. The transmission rate has remained stable with time and was not influenced by the mode of delivery, the mode of maternal infection, or the mother's ethnic origin. It was twice as high among the breast-fed infants as among the bottle-fed infants (40 vs. 19%, p < 0.04). Two factors were identified in a multivariate analysis (that did not include lymphocyte subset counts and the mode of feeding) as being associated with an increased risk of maternofetal transmission: p24 antigenemia (odds ratio = 3.1, confidence interval, = 1.5-6.2; p < 0.003) and elevated maternal age (p < 0.05). In the subgroup of 277 women whose absolute CD4+ lymphocyte counts at the time of delivery were available, the risk of transmission increased gradually from 15% of counts of > 600 CD4+ cells to 43% at counts of < 200. The risk of transmission was also related to the percentage of CD8+ cells, but each of the two factors seemed to play an independent role: the risk was lowest (12%) when the CD4+ cell count was > 500 and the proportion of CD8+ cells was < or = 40%, and was highest (50%) for values < 200 and > 40%.(ABSTRACT TRUNCATED AT 250 WORDS)