Urinary excretion rate and total clearances of albumin, IgG, IgA and alpha 1-microglobulin, together with selectivity index and proteinuria, were determined by computerized nephelometry in 187 IDDM and NIDDM diabetic out-patients and in 39 healthy subjects in order to perform a prompt clinical assessment of diabetic nephropathy. Significant correlations between nephelometric and RIA procedures were demonstrated for the urinary excretion of albumin (p < 0.001) and total IgG (p < 0.001) in diabetic patients and healthy subjects. Nephelometry allowed us to classify diabetic patients in different stages of nephropathy: non nephropathic, normoalbuminuric with hyperfiltration, with incipient (microalbuminuric) and overt nephropathy (macroalbuminuric). Thirty consecutive subjects were analyzed within 1 h from the beginning of the procedure. A normal tubular function was demonstrated in non nephropathic, hyperfiltering and in 34% of microalbuminuric diabetic patients. On the contrary, in 66% of microalbuminuric and in 93% of macroalbuminuric patients alpha 1-microglobulin urinary levels were found above the upper normal limit. Urinary excretion of IgA was significantly increased only in macroalbuminuric diabetic patients (p < 0.001); this marker might therefore characterise the stage of overt nephropathy. Computerized nephelometry can be considered as a prompt, reproducible and high sensitive approach in the clinical evaluation of proteinuria and tubular function in diabetic renal disease.