The relationship between arterial blood pressure and diabetic nephropathy was examined in 61 Type 1 (insulin-dependent) diabetic patients (22 females and 39 males). All patients fulfilled the following criteria: persistent proteinuria (greater than 0.5 g/day), onset of diabetes before 31 years of age, age less than 42 years, serum creatinine less than 130 mumol/l, and no antihypertensive treatment. Thirty Type 1 diabetic patients without persistent proteinuria but matched for sex, age, ideal body weight and duration of diabetes, and 30 healthy subjects matched for sex, age and ideal body weight were also studied as controls. The diabetic patients with persistent proteinuria had elevated blood pressures (146/96 +/- 17/10 mmHg, mean +/- SD) compared with 123/75 +/- 11/8 mmHg in diabetic patients without persistent proteinuria, and normal subjects (120/77 +/- 6/6 mmHg; p less than 0.001 for each). Diastolic blood pressure greater than or equal to 95 mmHg was found in 51% of the group with persistent proteinuria. Elevated arterial blood pressure is frequently present in young Type 1 diabetic patients with diabetic nephropathy and normal serum creatinine.