The relation between hypertension and diabetic nephropathy is complex. Nephropathy is probably involved in the elevated blood pressure found in diabetic patients. In maturity onset diabetes, patients may also have hypertension which is associated with obesity or essential hypertension. It has been suggested that in both types of diabetes, hypertension enhances the development of diabetic nephropathy. Moreover, an aggressive antihypertensive treatment seems able to reduce rate of decline in kidney function in insulin-dependent diabetic patients with patent nephropathy. In this work, creatinine clearance and microalbuminuria in 20 diabetic patients (mostly with maturity-onset-diabetes) with known moderate and effectively treated hypertension were therefore measured and the results were compared with those for 18 normotensive diabetic patients and 22 controls. Duration of diabetes was from one to 26 years (mean: 11 years) and duration of hypertension was from one to 35 years (mean: 10 years). Patients and controls had normal serum creatinine and proteinuria below 0.1 g/l. Microalbuminuria was measured by immunonephelometric assay using specific antiserum (sensitivity = 1.5 mg/l; intra and interassay coefficients: 6.5% and 8% respectively). The highest value was observed in hypertensive diabetic patients with retinopathy (group 1). But hypertensive patients without retinopathy (group 2) and normotensive patients also had significantly increased microalbuminuria. In group 1, microalbuminuria was significantly higher than in group 2. The creatinine clearance was reduced in groups 1 and 2 versus normotensive diabetics, but hypertensive patients were older.(ABSTRACT TRUNCATED AT 250 WORDS)