It has been suggested that the decrease in blood pressure during sleep is greater in patients with chronic hypertension than in preeclamptic patients. The DINAMAP (device for indirect noninvasive automatic mean arterial pressure) monitor was used to determine the mean arterial, systolic, and diastolic blood pressure, at 15-minute intervals, and 24 hours. Three groups of 15 patients each were studied: mildly preeclamptic, chronically hypertensive, and normotensive patients in the third trimester. In addition, 2 patients with severe preeclampsia were studied. The automatic readings were compared with auscultated blood pressures. There was short-term variation in the systolic and diastolic blood pressure of 20 to 40 and 15 to 30 mmHg, respectively, with consecutive 15-minute recordings. A decrease in blood pressure during sleep was seen in all groups, except in the 2 patients with severe preeclampsia. This fall varied between 5 and 20 mmHg systolic and 5 and 18 mmHg diastolic. The auscultated blood pressure differed from the automatic blood pressure by 5 to 40 mmHg systolic and 5 to 35 mmHg diastolic. In the clinical management of patients, these findings are significant, as they demonstrate short-term variation in blood pressure and differences in the auscultated versus automatic blood pressure readings. Also, changes in blood pressure occur during sleep; this may be important with regard to the timing and dose of medication given to pregnant patients with chronic hypertension or preeclampsia.