A multicenter population-based study was undertaken from 1983 to 1985 in 12 diabetes centers in Lombardy, in order to assess the prevalence of microalbuminuria and clinical proteinuria. Out of a total population of 17,704 diabetics, 1,155 were randomly selected within four strata of duration of disease and albumin excretion rate (AER) assessment was carried out in 541 unselected subjects. Albuminuria was evaluated from an overnight urine collection using a radioimmunoassay. The overall prevalence rates were 25.8% for microalbuminuria (30 less than or equal to AER less than 350 micrograms/min) and 3.0% for macroalbuminuria (AER less than or equal to 350 micrograms/min). In Type II diabetes the rate of microalbuminuria increases with duration of disease up to 10 years and then tends to level off. This could be due to the existence of two subgroups of diabetics with different life expectancy and different degree of risk of nephropathy. The Italian prevalence of microalbuminuria appears to be rather similar to that of other studies, while the prevalence of macroalbuminuria is quite low with respect to middle and northern Europe, indicating a rather slow rate of progression to clinical proteinuria. This could be explained on the basis of differing protein intakes. The cases with AER greater than or equal to 30 micrograms/min, greater than or equal to 70 micrograms/min, and greater than or equal to 350 micrograms/min have been compared with matched diabetic controls with AER less than or equal to 15 micrograms/min.(ABSTRACT TRUNCATED AT 250 WORDS)