The present study was designed to evaluate the frequency of an increase in the urinary albumin excretion rate (UAER) and the factors involved in this parameter in non-diabetic obese patients ; 122 non-diabetic obese patients were investigated. None had proteinuria or history of nephropathy or uropathy. Fourty of them had moderate hypertension. Compared with a group of 22 lean controls, UAER was significantly higher in the obese patients (19.0 +/- 2.0 (SEM) mg/24 h vs 3.2 +/- 0.6 mg/24 h, p < 0.001). UAER was elevated (> 20 mg/24 h) in 29 patients (23.7 %). Prevalence of microalbuminuria was not significantly different in hypertensive than in normotensive patients. However UAER was significantly higher in the 32 patients with a family history of hypertension (29.6 +/- 6.3 mg/24 h vs 15.3 +/- 1.5 mg/24 h, p < 0.002). In patients with microalbuminuria, body weight was significantly higher (100.3 +/- 3.9 kg vs 91.8 +/- 1.9 kg, p < 0.05), plasma albumin was significantly lower (38.3 +/- 0.6 g/l vs 40.3 +/- 0.3 g/l, p <0.005) and the estimated value of fractional albumin clearance was significantly higher. These results show the high frequency of microalbuminuria in non-diabetic obese patients. They suggest that UAER level may be an index of family hypertension in obese patients and that microalbuminuria is part of a widespread abnormality of the capillary permeability.