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Background and design: The effects of aspirin on blood pressure (BP) are controversial and a chronopharmacological effect of aspirin on 24-hour BP was reported recently in otherwise untreated hypertensive patients. The study was designed to test the timing effect of aspirin dosing on 24-hour BP in treated hypertensive patients routinely taking aspirin for cardiovascular prevention.Method and results: Seventy-five patients were randomized into two groups. One group was to receive aspirin in the evening then in the morning for 1 month and the other group in the morning then in the evening, following a cross-over design. The principal assessment criterion was 24-hour systolic BP (SBP) measured by 24-hour ambulatory BP monitoring (ABPM). Patients were aged 65 ± 9 years and had been hypertensive for 12 ± 10 years. They were all taking a mean of 2.8 antihypertensive drugs and did not modify their treatment throughout the study. Of the included subjects, 70% were men and 33% were diabetics. Mean 24-hour SBP values were clinically equivalent and were not statistically different, depending on whether the aspirin was taking in the morning or evening (128.3 ± 1.4 vs. 128.3 ± 1.4 mmHg, respectively). Neither was there any significant difference in diurnal and nocturnal SBP or in 24-hour, diurnal, and nocturnal diastolic BP (DBP).Conclusion: It does not appear useful to advise patients with long-standing hypertension to modify timing of aspirin intake in order to reduce BP values.