One hundred thirty-one nulliparous patients between 28 to 34 weeks' gestation with no known history of renal disease or hypertension were placed in the left lateral recumbent position, and their blood pressures were taken once and recorded. They then turned to the supine position where another blood pressure was taken within 2 minutes and also recorded. All these patients were then followed routinely through their pregnancy, labor, delivery, and postpartum period. Of those patients whose diastolic blood pressure during the modified “roll-over” test rose less than IS mmHg, only 2 of 99 developed pregnancy-induced hypertension (PIH). Of those whose roll-over test showed an increase of more than 15 mmHg, 23 of 32 developed PIH (P<.000000001). Of those whose blood pressure during the roll-over test increased greater than or equal to 20 mmHg, 19 of 25 developed PIH (P<.00000001). Also, 10 of 11 patients whose diastolic blood pressure increased more than 25 mmHg during the roll-over test developed PIH (P<.000000001).