Microalbuminuria has been established as a marker strongly predictive of diabetic nephropathy. The appearance of microalbuminuria (30-150 micrograms/min) is considered to herald incipient nephropathy, and the progression to clinical proteinuria (greater than 200 mg/24 h) is believed to reflect a shift from reversible to irreversible renal damage in diabetic patients. Periodic monitoring of albumin excretion could thus detect diabetic renal disease at an early, potentially reversible stage. However, this is not routinely done, largely due to cumbersome collection and methodologic constraints. We therefore have developed a simplified competitive immunoassay (ELISA) that is sensitive to 10 ng and reproducibly quantifies urinary albumin levels between 0.1-10 micrograms/ml, a range appropriate to that demarcating normal from microalbuminuric patients. Examination of results obtained with aliquots of 24 h and fractional urine collections, without and with correction for creatinine (albumin: creatine ratio), in basal and post-exercise states indicates that (a) this assay can effectively measure urine albumin concentration in single void specimens, and albumin excretion rates in fractional collections, and (b) conversion to albumin:creatinine ratio is not necessary to discriminate normal from microalbuminuric states.