Intensive monitoring of adherence in patients with uncontrolled hypertension was evaluated over a 6-month period. After that period, only patients well characterized as having resistant hypertension were followed for 12 months. The goal of this study was to evaluate whether adherence to a drug regimen helps to identify patients with resistant hypertension. Forty-four hypertensive patients resistant to a 3-drug regimen (average blood pressure [BP] mm Hg, mean +/- standard deviation) were studied prospectively. Each patient was followed for a 12-month period. Adherence to treatment was evaluated through self-report, applying Morisky's questionnaire and the pill count method. Ambulatory BP monitoring and office BP measures were performed. By pill count, 63.6% of the patients were adherent to treatment at the start of the survey and 94% at the end, although 59% of the patients still did not reach normal BP levels. We found that non-adherence was not associated with resistance to antihypertensive treatment. Therefore, after investigation, we concluded that patients who presented with uncontrolled arterial BP may be truly resistant hypertensive to treatment.