Twenty-one diabetic males, ages 20 to 61 years (mean 48.4 +/- 10.5) and 14 healthy males, 22 to 59 years (mean 42.5 +/- 10.4) consented to participate in a study during which 24-h recordings of systolic and diastolic blood pressures, heart rates, and rhythm were obtained. The diabetic subjects were considered normotensive except for two patients, who had been observed to have rare insignificant elevations in blood pressure and were untreated. Diabetic subjects had a higher mean maximal systolic blood pressure (160.7 +/- 49.8 mmHg) than the normal controls (132.4 +/- 12.1 mmHg) (p less than 0.05). They also had a higher frequency (15.1%) of systolic blood pressure readings greater than or equal to 150 mmHg compared to normal men (0.2%) (p less than 0.01). The frequency of diastolic blood pressure readings greater than or equal to 90 mmHg was 22.1% in the former and 9.2% in the latter group (p less than 0.01). Of 21 diabetic males, 14 (66.7%) had systolic pressures greater than or equal to 150 mmHg, diastolic pressures greater than or equal to 100 mmHg, or both. In the normal men, 3 (21.4%) of the 14 had such pressures. Twenty-four hour monitoring of ambulatory blood pressures revealed elevations not detected by routine casual readings in patients with diabetes.