A blood pressure surge during the early morning may help to precipitate cardiovascular events. The objective of this study was thus to assess the blood pressure behaviour profile of early morning blood pressure in patients receiving antihypertensive treatment. The ACAMPA study is a multi-center, open, prospective, observational study that was carried out by 24 investigators in Spain. Two hundred and ninety patients with essential hypertension who had been receiving the same antihypertensive treatment for at least 2 months were included in the study. Office blood pressure was measured before taking medication in the morning, and 24-h ambulatory blood pressure monitoring was performed. In addition to the automatic measurements, patients were instructed to take a blood pressure measurement after waking. The group analysis used 240 patients (mean age 54 years, including 101 males). Good clinical control (a blood pressure of less than 140/90 mmHg) was found in 53 cases (22%). The differences between the clinical and ambulatory readings during the period of activity were minimal in the group with good control (127 +/- 9/81 +/- 7 versus 127 +/- 10/81 +/- 7 mmHg; non-significant) but were significant in the group with poor control (155 +/- 16/93 +/- 10 versus 138 +/- 14/86 +/- 11 mmHg; P < 0.001). The blood pressure values were synchronized according to the moment of awakening. In almost half of the patients with good control, systolic and diastolic blood pressure values were higher than normal (135/85 mmHg); in those patients with poor control, this fraction rose to over 70%. The proportion of patients presenting high blood pressure values became even greater during the second hour after waking (62% in patients with good blood pressure control and 82% in those with poor control). In a large number of antihypertensive patients receiving treatment, blood pressure values remain high during the early-morning hours. At least half of those patients with an apparently well-controlled office blood pressure do not have their blood pressure under control for the period shortly after waking.