BackgroundGestational hypertension is one of the three common causes of maternal death. This study was designed to compare the effects of aspirin and placebo on 24 hour blood pressure mean in women at risk of preeclampsia in two different diurnal times to prevent preeclampsia. MethodsSixty four women who were at risk of gestational hypertension were recruited to be included in the study using convenience sampling method and divided into 4 groups by random allocation. Control groups one and two received placebo tablet. Experimental groups one and two received aspirin tablets (100 mg) in the morning and at night, respectively , one tablet a day every day starting at the time of recruitment and continued till 32nd week of gestation, by double blind method. The patients’ blood pressure was monitored at the end of each month for 24 consecutive hours from 12 to16 weeks until delivery. Data were analyzed using repeated measures design and along with multiple comparative tests in SPSS software (version 8.0). ResultsIn Aspirin and control group 1, systolic and diastolic BP mean variations were similar, (P=0.835 and P=0.705 for systole and diastole, respectively). In Aspirin and control group 2, systolic and diastolic BP mean variations were statistically significant. (for systole and diastole P<0.001). Mean reduction for systole was 14.12 mmHg in 32nd week and 12.12 mmHg in 40th week and for diastole it was 11.69 mmHg in 32nd week and 9.04 mmHg in 40th week.ConclusionTaking aspirin at night is more effective on 24-hour blood pressure mean reduction in women at preeclampsia risk compared with that in the morning. Trial Registration Number: IRCT138803211548N5