Objective: Our purpose was to determine whether in a low-dose aspirin trial a longitudinal decrease in maternal serum thromboxane B2 is associated with improvement in pregnancy outcomes. Study Design: a total of 606 healthy nulliparous women with singleton gestations were randomized at 24 weeks to either 60 mg of aspirin or a placebo. Maternal serum thromboxane B2 was measured at randomization, at 29 to 31 weeks, at 34 to 36 weeks, and at delivery. After delivery, and without knowledge of patient outcome or group assignment, patients were categorized as having had either a longitudinal twofold or greater (≥50%) or less than twofold reduction (<50%) in thromboxane B2 from baseline levels at randomization. Results: Of 606 entrants, 92% had sufficient thromboxane B2 determinations to allow categorizatino. Whether patients were assigned to aspirin or placebo, birth weight was significantly greater in women who had a twofold or greater reduction in maternal serum thromboxane B2 levels. When the aspirin and placebo groups were combined, women with a twofold or greater reduction in thromboxane B2 levels had less preeclampsia, 1.9% (6/314) versus 5.7% (14/244) (p = 0.016), less preterm delivery (5.7% vs 10.7%, p = 0.032), fewer small-for-gestational-age newborns, 9 of 314 (2.95) versus 17 of 244 (7%) (p = 0.023). and a higher mean birth weight, 3314 gm versus 3121 gm(p = 0.0001). Conclusion: Women with a twofold or greater longitudinal reduction in maternal serum thromboxane B2 had less preeclampsia and prematurity, fewer small-for-gestational-age newborns, and higher birth weights than women with less than a twofold reduction. © 1995, All rights reserved.