Recognition of hypertension and abnormal blood pressure burden with ambulatory blood pressure recordings in type I diabetes mellitus. Ambulatory blood pressure (AMBP) measurements were obtained at 20-min intervals for 24 h in 25 subjects with insulin-dependent (type I) diabetes mellitus and 21 control subjects. The diabetic patients had normal kidney function (glomerular filtration rate 112.1 +/- 7.2 ml.min-1.1.73 m-2, renal plasma flow 459.0 +/- 23.4 ml.min-1.1.73 m-2) and were normotensive according to standard sphygmomanometer examinations. Mean +/- SE AMBP (systolic/diastolic in mmHg) measurements in diabetic patients (24 h, 131.7/77.2 +/- 2.9/1.8; 0600-2200, 132.3/78.4 +/- 2.9/3.4; 2200-0600, 125.1/75.7 +/- 3.9/3.4) significantly exceeded control values during all times (24 h, 121.8/70.3 +/- 2.9/1.9; 0600-2200, 120.7/71.8 +/- 2.6/2.0; 2200-0600, 108.2/61.5 +/- 6.6/2.7). Mean 24-h AMBP exceeded 135/85 mmHg in 49% of diabetic patients. The same threshold of 135/85 mmHg was used to determine the prevalence of abnormal measurements per time period (pressure burden). Pressure burden was increased twofold in diabetic patients compared with control subjects. Mean AMBP was significantly reduced at night in control subjects but not in diabetic patients. Changes in blood pressure were not related to kidney function in diabetic patients. AMBP recordings uncovered an increased prevalence of abnormal mean blood pressure, increased pressure burden, and a lack of diurnal variation of blood pressure in subjects with type I diabetes mellitus. These findings have important implications for early intervention strategies in diabetes mellitus because AMBP recordings correlate well with end-organ damage. 