The purpose of this investigation was to determine the diagnostic value of an average mean arterial blood pressure in the second trimester of greater than or equal to 90 mm Hg and a threshold increase in diastolic blood pressure of greater than or equal to 15 mm Hg or in systolic blood pressure of greater than or equal to 30 mm Hg, on two occasions, 6 hours apart, in predicting preeclampsia. The study population consisted of 700 young normotensive primigravid women who were evaluated prospectively during pregnancy. Systolic and diastolic blood pressures were carefully measured at each prenatal visit, and the mean arterial blood pressure in the second trimester was calculated for each measurement. An average greater than 90 mm Hg was considered abnormal. One hundred thirty-seven patients had preeclampsia, for an overall incidence of 19.6%. An average greater than 90 mm Hg had a sensitivity of 8% and a positive predictive value of 23%. The respective values for a threshold increase of greater than 15 mm Hg in diastolic pressure were 39% and 32%. For a threshold increase of greater than 30 mm Hg in systolic pressure, values were 22% and 33%. The negative predictive values for all tests studied ranged between 81% and 85%. Neither a mean arterial blood pressure in the second trimester of greater than 90 mm Hg nor a threshold increase in systolic or diastolic blood pressure during the third trimester was significantly predictive of the development of preeclampsia.