This study assesses differences in blood pressure (BP) levels prospectively between office (manual) measurement and ambulatory blood pressure monitoring (ABPM) in 70 treated, essential, hypertensive patients. The objective was to determine whether ABPM is superior to office measurement for assessing adequacy of therapy. Twenty-four patients received monotherapy and 46 received multiple therapy. Thirty-five patients were administered medication in the morning only, whereas 33 were administered medication in the morning and evening both. Mean systolic BP by manual method was identical to that obtained by ABPM (141.98 +/- 14.98 mm Hg versus 141.46 +/- 16.33 mm Hg, respectively). However, mean diastolic BP by manual method was significantly higher than that obtained by ABPM. (90.38 +/- 9.01 mm Hg versus 86.69 +/- 10.65 mm Hg, respectively; P < 0.001). Significant correlations (P < 0.01) were found between the BP levels measured by these two methods, although individual readings differed by 10 mm Hg or more systolic and by 5 mm or more diastolic in many subjects. No significant differences were noted in BP levels measured by either method for patients treated by monotherapy or multiple therapy, and none were noted whether they were taking medication in the morning or in both the morning and evening. In addition, no differences were noted in BP levels using either method for race. Thus, this study shows that the office measurement is grossly similar to ABPM for assessment of adequacy of therapy in treated hypertensive patients whose blood pressure is controlled adequately. However, ABPM is found to be superior to office measurement in identifying hypertensive patients whose blood pressure is not controlled adequately or is uncontrolled.