We studied the effectiveness of blood pressure (BP) control outside the clinic by using ambulatory BP monitoring (ABPM) among a large number of hypertensive subjects treated in primary care centers across Spain. The sample consisted of 12 897 treated hypertensive subjects who had indications for ABPM. Office-based BP was calculated as the average of 2 readings. Twenty-four-hour ABPM was then performed using a SpaceLabs 90207 monitor under standardized conditions. A total of 3047 patients (23.6%) had their office BP controlled, and 6657 (51.6%) were controlled according to daytime ABPM. The proportion of office resistance or underestimation of patients' BP control by physicians in the office (office BP >or=140/90 mm Hg and average daytime ambulatory BP <135/85 mm Hg) was 33.4%, and the proportion of isolated office control or overestimation of control (office BP <140/90 mm Hg and average daytime ambulatory BP >or=135/85 mm Hg) was 5.4%. BP control was more frequently underestimated in patients who were older, female, obese, or with morning BP determination than in their counterparts. BP control was more frequently overestimated in those who were younger, male, nonobese, smokers, or with evening BP determination. Ambulatory-based hypertension control was far better than office-based hypertension control. This conveys an encouraging message to clinicians, namely that they are actually doing better than is evidenced by office-based data. However, the burden of underestimation and overestimation of BP control at the office is still remarkable. Physicians should be aware that the likelihood of misestimating BP control is higher in some hypertensive subjects.