We identified 19 women who had persistently positive test results for antiphospholipid antibodies who were considered to be at low risk because they had none of the associated signs or symptoms of the antiphospholipid antibody syndrome. They had had no (10/19, 53%) or just one prior spontaneous abortion and did not have a history of thrombosis or thrombocytopenia. Many (8/19, 42%) had had a prior uncomplicated pregnancy ending in a live birth. These women were randomly assigned to receive low-dose aspirin (81 mg daily) or usual care. There were few obstetric complications recorded in either treatment group. One woman in the aspirin group had a fetal death, and one in the usual care group had a low-birth-weight infant. The frequency of complications was so low that >600 such women would need to be entered into a randomized trial to evaluate whether low-dose aspirin would be beneficial treatment during a pregnancy. We concluded that treatment of pregnant women with antiphospholipid antibodies who are otherwise at low risk cannot be justified on the basis of the available evidence. (Am J Obstet Gynecol 1997;176:1099-100.)