Diabetic patients with elevated urinary albumin excretion rate (incipient or clinical nephropathy) also have an increased transcapillary escape rate of albumin. This study was designed to clarify whether this is caused by a general vascular dysfunction or by elevated systemic blood pressure. The systemic blood pressure and the transcapillary escape rate of albumin were measured in the following groups after 4 weeks without antihypertensive treatment: Group 1--eleven healthy control subjects. Group 2--ten Type 1 (insulin-dependent) diabetic patients with incipient nephropathy (urinary albumin excretion rate: 30-300 mg/24 h) and normal blood pressure. Group 3--eleven non-diabetic patients with essential hypertension. Group 4--nine Type 1 diabetic patients with hypertension but normal urinary albumin excretion (< 30 mg/24 h). Group 5--eleven Type 1 diabetic patients with nephropathy (urinary albumin excretion rate > 300 mg/24 h) and hypertension. Systolic and diastolic blood pressure were similar in the three hypertensive groups: group 3, 148 +/- 8/95 +/- 6; group 4, 150 +/- 12/94 +/- 8 and group 5; 152 +/- 12/92 +/- 7 mmHg, but significantly elevated (p < 0.001) compared to control group 1, 117 +/- 12/74 +/- 9 and group 2, 128 +/- 7/82 +/- 4 mmHg. The transcapillary escape rate of albumin was similar in the control subjects (5.2 +/- 2.7%) and the subjects in the normoalbuminuric groups 3 and 4 (6.2 +/- 1.9 and 5.1 +/- 1.4%, respectively) and significantly lower (p < 0.001) than in patients with elevated urinary albumin excretion without or with hypertension group 2, 10.1 +/- 2.8 and group 5, 11.4 +/- 5.7%.(ABSTRACT TRUNCATED AT 250 WORDS)