Objective: The aim of the study was too determine whether, in a low-dose aspirin trial in high-risk pregnancies, a decrease in maternal serum thromboxane B₂ level predicted improved pregnancy outcomes. Study Design: This multicenter, randomized, double-blind trial included 2539 women, 1010 of whom had sufficient serum samples at enrollment and at 24 to 28 weeks’ gestation, 34 to 38 weeks’ gestation, or both to assess longitudinal changes in thromboxane B₂ level and their effects on pregnancy outcomes. Women were randomly assigned between 13 and 26 weeks’ gestation to receive daily aspirin (60 mg) or placebo. Results: Overall and in all subgroups women assigned to receive aspirin had markedly lower maternal thromboxane B₂ concentration values than did those assigned to receive a placebo (P = .0001). Changes in thromboxane levels were not, however, correlated with adverse pregnancy outcomes. Women with ≥50% reduction in maternal serum thromboxane B₂ concentrations from baseline had occurrences of preeclampsia (P = .922), preterm birth (P = .375), small for gestational age neonates (P = .938), and grade III or IV intraventricular hemorrhage (P = 1.000) similar to those of women who had <50% reduction. Similar results were found for women with thromboxane B₂ level decreases of <15 versus >15 ng/mL and women with thromboxane B₂ level decreases to <10 versus ≥10, <5 versus ≥5, and <1 versus ≥1 ng/mL. Maternal thromboxane B₂ concentrations at enrollment were also not predictive of adverse outcomes. Conclusion: Neither maternal serum thromboxane B₂ concentrations at enrollment nor their subsequent reduction were predictive of adverse pregnancy outcomes in a low-dose aspirin trial. (Am J Obstet Gynecol 1998;179:1193-9.)