ObjectiveThe Hypertension Optimal Treatment (HOT) study is a large, prospective trial aimed at defining the level of diastolic blood pressure required during antihypertensive therapy in order to achieve maximal protection against cardiovascular complications. A further aim is to assess the effects on morbidity and mortality of a 75 mg daily dose of aspirin compared with placebo.Subjects and methodsCompliance with double-blind administration of aspirin or placebo added to antihypertensive treatment was evaluated for 1 year in a subset (n= 530) of the study population (n= 18790) by placing the medication in a container closed with an electronic cap that records precisely the time of each opening.ResultsThe 1-year compliance rate (percentage of days with one opening per day) could be assessed in 501 patients. It averaged 78.3 ± 25% in aspirin-treated patients (n= 236, mean ± SD), compared with 78.5 ± 25% in patients having received placebo (n= 265), and was not influenced by age, sex or country (Germany, Italy, Switzerland, UK). The compliance rate was also similar irrespective of whether the patients had reached their target blood pressure, but was significantly better during the first than the second 6-month monitoring period (84.1 ± 22% versus 72.3 ± 32%,n= 501).ConclusionsThe high rate of compliance with aspirin or placebo observed in the HOT study suggests that the patients were highly motivated and may account for the unusually good blood pressure control achieved in this trial during long-term anti-hypertensive treatment.J Hypertens1999, 17:1041–1045 © Lippincott Williams & Wilkins.