We designed a cross-sectional study to determine whether 6-item self-administered questionnaires addressing difficulties in taking treatment provide independent and relevant information on uncontrolled hypertension in high-risk cardiovascular patients seen in general practice. Patients with both treated hypertension and a history of vascular diseases-myocardial infarction, stroke, or peripheral artery disease-were included. Risk factors, treatment, history of vascular diseases, blood pressure, and difficulties in taking treatment were assessed by 6-item self-administered questionnaires and recorded. Each positive response to the questions was weighted by 1 and each negative response by 0. Individual item scores were added together to produce 1 composite score for all 6 questions. A total of 11,096 patients were analyzed. Among them, 5,288 (51.4%) were controlled at 140/90 mm Hg threshold. In multivariate analysis, in addition to age, male gender, treated diabetes, peripheral artery disease, treatment, and alcohol consumption, the adherence score was negatively and independently associated with hypertension control (odds ratio score >/= 3, 0.73; [95% confidence interval, 0.65-0.81; P < .0001]. This study overwhelmingly confirms on a very large scale the effectiveness of this self-administered questionnaire in identifying difficulties in taking treatment in general practice. This questionnaire constitutes an inexpensive and timesaving tool capable of helping general practitioners to understand why hypertension is not controlled in patients at high cardiovascular risk. Whether the use of this questionnaire will improve hypertension control remains to be established.