Ambulatory blood pressure, ABP, was determined every 15 min for 24 h (Spacelabs 5200 system) in 16 hypertensive diabetic subjects treated for high blood pressure. Office blood pressure (OBP) in these subjects (systolic BP greater than 160 mmHg and diastolic BP greater than 95 mmHg before treatment) had been reduced by treatment to the borderline range (systolic less than or equal to 160 mmHg and/or diastolic less than or equal to 95 mmHg). Sixty-five diabetic subjects with normal or borderline OBP were included as controls. The two groups had the same age (58 +/- 10 yrs in both groups), duration of diabetes (15 +/- 9 yrs), 24 hr microalbumin, and included the same percentage of subjects with moderate neuropathy (36% and 29%, NS). The two groups had the same OBP (138 +/- 16 mmHg and 140 +/- 16 mmHg systolic, NS, 84 +/- 9 mmHg and 84 +/- 13 mmHg diastolic, NS). In contrast, ambulatory BP was significantly higher in the treated group, when compared with the controls (123 +/- 13 mmHg and 133 +/- 23 mmHg systolic, P less than 0.025, 77 +/- 7 mmHg and 84 +/- 16 mmHg diastolic, P less than 0.015). The difference was significant both in daytime and in nighttime, and was more significant in nighttime (11 mmHg systolic, P less than 0.02, 9 mmHg diastolic, P less than 0.004) than in daytime (9 mmHg systolic, P less than 0.05 and 5 mmHg diastolic, P less than 0.05). Ambulatory heart rate was also significantly higher in the treated group, but only in daytime (7 b/min difference, P less than 0.02). The study demonstrated the need to survey and investigate ABP in treated hypertensive diabetic subjects.