Nonsteroidal antiinflammatory drugs may affect blood pressure (BP) control in hypertensive patients receiving drug treatment, but data on the effects of low-dose aspirin are scanty. This study assessed the effects of chronic treatment with low doses of aspirin (100 mg/day) on clinic and ambulatory systolic (SBP) and diastolic (DBP) BP in hypertensives on chronic, stable antihypertensive therapy. The study was conducted in the framework of the Primary Prevention Project (PPP), a randomized, controlled factorial trial on the preventive effect of aspirin or vitamin E in people with one or more cardiovascular risk factors. Fifteen Italian hypertension units studied 142 hypertensive patients (76 men, 66 women; mean age 59 +/- 5.9 years) treated with different antihypertensive drugs: 71 patients were randomized to aspirin and 71 served as controls. All patients underwent a clinic BP evaluation with an automatic sphygmomanometer and a 24-h ambulatory BP monitoring, at baseline and after 3 months of aspirin treatment. At the end of the study the changes in clinic SBP and DBP were not statistically different in treated and untreated subjects. Ambulatory SBP and DBP after 3 months of aspirin treatment were similar to baseline: deltaSBP -0.5 mmHg (95% confidence intervals [CI] from -1.9 to +2.9 mm Hg) and deltaDBP -1.1 mm Hg (95% CI from -2.5 to +0.3 mm Hg). The pattern was similar in the control group. No interaction was found between aspirin and the most used antihypertensive drug classes (angiotensin converting enzyme inhibitors and calcium antagonists). Despite the relatively small sample size our results seem to exclude any significant influence of low-dose aspirin on BP control in hypertensives under treatment.