Pre-eclampsia is a common and serious complication of pregnancy that affects both mother and child. Review of previous small trials of antiplatelet therapy, particularly low-dose aspirin, suggested reductions of about three-quarters in the incidence of pre-eclampsia and some avoidance of intrauterine growth retardation (IUGR), but larger trials have not confirmed these results. In our multicentre study 9364 women were randomly assigned 60 mg aspirin daily or matching placebo. 74% were entered for prophylaxis of pre-eclampsia, 12% for prophylaxis of IUGR, 12% for treatment of pre-eclampsia, and 3% for treatment of IUGR. Overall, the use of aspirin was associated with a reduction of only 12% in the incidence of proteinuric pre-eclampsia, which was not significant. Nor was there any significant effect on the incidence of IUGR or of stillbirth and neonatal death. Aspirin did, however, significantly reduce the likelihood of preterm delivery (19.7% aspirin vs 22.2% control; absolute reduction of 2.5 [SD 0.9] per 100 women treated; 2p = 0.003). There was a significant trend (p = 0.004) towards progressively greater reductions in proteinuric pre-eclampsia the more preterm the delivery. Aspirin was not associated with a significant increase in placental haemorrhages or in bleeding during preparation for epidural anaesthesia, but there was a slight increase in use of blood transfusion after delivery. Low-dose aspirin was generally safe for the fetus and newborn infant, with no evidence of an increased likelihood of bleeding. Our findings do not support routine prophylactic or therapeutic administration of antiplatelet therapy in pregnancy to all women at increased risk of pre-eclampsia or IUGR. Low-dose aspirin may be justified in women judged to be especially liable to early-onset pre-eclampsia severe enough to need very preterm delivery. In such women it seems appropriate to start low-dose aspirin prophylactically early in the second trimester.