Human immunodeficiency virus (HIV) has been isolated from fetal tissues as early as 13 weeks and later from fetal blood. These findings have raised the possibility of prenatal diagnosis of infected fetuses by identification of the virus in the fetal compartment. Study of the fetal immune status has proved reliable in prenatal diagnosis of congenital immunodeficiency, and we have tested the possibility to diagnose acquired immunodeficiency in utero by this approach. We studied T lymphocyte subsets and their mitogenic response in fetal blood obtained after elective termination at midgestation in 8 cases and at delivery in 26 cases of maternal HIV infection. Results have been compared to appropriate normal controls. No significant difference was found in terms of total lymphocytes, CD4 and CD8 populations and phytohemagglutinin responses. This indicates either that immunological parameters currently used to assess postnatal immunodeficiency are not reliable during intrauterine life or that the intrauterine environment and the transplacental passage of maternal antibodies interfere with development of prenatal immunodeficiency.