To investigate the effect of maternal HLA antibodies on the evolution of pregnancy, we tested sera from 187 pregnant women for the presence of lymphocytotoxic antibodies (anti-HLA-A, -B, and -C and anti-B lymphocytes or anti- HLA-DR). Patients were studied before 20 weeks, between 21 and 30 weeks, and between 31 and 40 weeks' gestation. No correlation was found between the presence of such antibodies and obstetric complications, fetal wastage, placental weight, or infant birth weight. Patients with anti-DR antibodies delivered more female than male infants by a 3:1 ratio.