A prospective randomized double blind study with processed rhubarb (low dose of 0.75g/day) was carried out in pregnant women at risk of pregnancy induced hypertension (PIH). Rhubarb (140 cases) or placebo (125 cases) was given to women at risk of PIH consecutively from the 28th week of gestation till delivery, and another 68 pregnant women as control. Results showed that 5.7% of rhubarb treated women developed PIH, a rate substantially lower than the 20.8% of the placebo group (P < 0.01). After 9-10 weeks of treatment, the plasma fibronectin (Fn) level and Plasminogen activator inhibiter (PAI) value were found significantly lower (P < 0.05) in the rhubarb treated group than in the placebo. Antithrombin III (ATIII) level also decreased significantly less in the rhubarb group as compared with the placebo (P < 0.05). It indicated that low dose of processed rhubarb has a good prophylactic effect on PIH. The mechanism may be related with the inhibition of PAI activity, reduced Fn synthesis and decreased damage to vascular endothelium.