To identify factors related to poor control of blood pressure in primary care, we designed a retrospective case-control analysis of clinical and demographic data recorded in the General Practitioners (GP) database. Study data were provided on a voluntary basis by 21 GPs from a practice-based network in primary care. The study included 2519 hypertensive patients enrolled between January 1 and December 31, 2000. The interventions were antihypertensive medication, and the main outcome measures were control of systolic and diastolic blood pressure (BP). The independent variables considered were: age of patient and GP; patient gender, body mass index, history of smoking, diabetes mellitus, or cholesterol tests; family history of hypertension; previous visits for cardiologic, nephrologic, or vascular surgery evaluation; prior hospitalizations for myocardial infarction or heart failure, and number of admissions for surgery; length of patient follow-up, type of antihypertensive medication, mean daily dosage, adherence to the drug regimen, and number of other medications currently being taken by the patient. Blood pressure was uncontrolled (>140/90 mmHg) in 1525 (60%) of the 2519 hypertensive patients enrolled. The presence of diabetes mellitus, increasing patient age, and increasing GP age significantly increased the risk of uncontrolled BP. Factors significantly associated with a reduced risk of uncontrolled BP were the number of other medications currently being taken by the patient and a prior history of MI. We conclude that the failure of antihypertensive medication to adequately control BP is determined by both the patient's characteristics and factors related to the patient-doctor relationship. Successful treatment of hypertension requires patient adherence to the regimen that has been agreed on by the patient and the physician.