Objective: The small controlled trials reporting large reductions in the incidence of preeclampsia and intrauterine growth restriction (IUGR) in high-risk pregnant women treated with low-dose aspirin have recently been followed by large clinical trials suggesting less beneficial results. The effect of low-dose aspirin on placental lesions associated with preeclampsia and IUGR has not yet been studied. Methods: We participated in the large multicenter randomized collaborative low-dose aspirin study in pregnancy (CLASP) trial of low-dose aspirin for the prevention and treatment of preeclampsia and intrauterine growth restriction. As part of this study, we evaluated placentae submitted from 25 women treated with aspirin and 28 with placebo. Results: More of the pathological findings classically described in preeclampsia and IUGR were demonstrated in the placentae from the placebo group than from the aspirin group (54% vs. 16%, P = 0.02). The placental findings did not correlate with clinical pregnancy outcome or Doppler flow parameters of the fetal umbilical artery in either group. Conclusions: Our results support the assumption that aspirin may have some inhibitory effect on the uteroplacental circulatory ischemic changes typically occurring in preeclampsia and IUGR.