Urinary excretion of albumin was determined in 133 clinically healthy subjects and 236 patients suffering from diabetes mellitus (150 cases), collagen diseases (45 cases), or arterial hypertension (41 cases) in whom there was no clinical or laboratory evidence of renal involvement. Urine albumin was measured by an improved single-radial-immunodiffusion technique and results were expressed as urine albumin to creatinine concentration ratio (mg/g). The geometric mean values in the patients were about 2 to 2.5 times higher than in controls. When considered on an individual basis, at least 40 per cent of the patients had values above the 99th-percentile value for controls. The results suggest that quantitative determination of urinary albumin may be useful for detecting early structural functional alterations of the glomerular filter.