To analyze determinants of the risk of pregnancy-induced hypertension (PIH) with or without proteinuria, we compared characteristics of women enrolled in the Italian Study of Aspirin in Pregnancy who developed PIH and those who did not. A total of 756 women were included in the present analysis; of these, 132 women (17%) developed PIH during the trial. The risk of developing PIH tended to increase with maternal age: in comparison with women age 20-25 years, the odds ratio (OR) estimates of risk ratio were 3.5 [95% confidence interval (CI) = 1.6-7.1] in women age 26-30 years and 4.2 (95% CI = 1.9-8.8) in those age > 30 years. There was little relation between development of PIH and education. PIH risk increased according to nonpregnant body mass index; in comparison with women with Quetelet's index (kg per m2) < 25, the OR estimates were 1.7 (95% CI = 1.1-2.7) and 2.1 (95% CI = 1.3-3.6), respectively, for women with a value for Quetelet's index of > 25-30 and > 30. Parous women were at decreased risk of PIH: in comparison with nulliparas, the ORs were 0.7 (95% CI = 0.4-1.0) and 0.5 (95% CI = 0.3-0.9), respectively, in women reporting 1 or > or = 2 births. There was no important relation between previous spontaneous or induced abortion and PIH risk.