Lupus anticoagulant, anticardiolipin, antinuclear, anti-deoxyribonucleic acid, antithyroglobulin, and antithyroid microsomal antibodies were assayed during third-trimester pregnancy (100 normal, 100 with complications). In spite of a normal activated partial thromboplastin time in all instances, lupus anticoagulant was further investigated by three additional procedures: tissue thromboplastin inhibition time, platelet neutralization procedure, and cephalin neutralization test. The prevalence of autoantibodies in pregnancies with hypertension reaches 16% (four with lupus anticoagulant, two with anticardiolipin, and two with antithyroid microsomal antibodies), which is significantly greater than that for idiopathic fetal growth retardation (2%) (one with lupus anticoagulant antibodies) and normal pregnancies (3%) (two with antithyroglobulin and one with autithyroid microsomal antibodies) (p < 0.01). Autoantibodies were equally distributed between patients with gestational hypertension and those with preeclampsia. When compared with the 42 patients with hypertension and no autoantibodies, the eight patients with autoantibody had a more frequent history of fetal growth retardation (p < 0.05), but there was no difference in the severity of hypertension, the frequency of obstetric complications, or the outcome of pregnancy. They did not require any specific treatment.