A prospective randomized double-blind study was carried out in pregnant women with risk of pregnancy induced hypertension (PIH). Low dose Aspirin (50 mg/day) or placebo was given consecutively from the 28th weeks of gestation. The results have shown that 8% of the pregnant women in the aspirin treatment group had developed PIH, which was substantially lower than that in the control group (24%) (P less than 0.05). The ratio of TXB2/6-keto-PGF1 alpha increased significantly in the control group while it remained unchanged in treatment group. Increasing plasma fibronectin (Fn) and decreasing AT-III level were seen in the control group but no changes of these parameters in the treatment group. It was presumed that low dose aspirin may have prophylactic effect on PIH. The mechanism of aspirin may be the inhibition of TXA2 and Fn synthesis and decreased consumption of AT-III.