There are few population-based epidemiological data describing the long-term incidence of gross proteinuria in people with diabetes. We performed a population-based study in southern Wisconsin of individuals with diabetes diagnosed at either < 30 years of age and taking insulin (younger-onset, n = 666) or > or = 30 years of age either taking (older-onset taking insulin, n = 376) or not taking insulin (older-onset not taking insulin, n = 418). The presence of gross proteinuria (> or = 0.3 g/l) was determined by means of a reagent strip. The incidence of proteinuria in the 10-year interval was 28% in the younger-onset group, it was 40% in the older-onset group taking insulin, and it was 33% in the older-onset group not taking insulin. After we controlled for other risk factors, the 10-year incidence of proteinuria was significantly related to higher glycosylated hemoglobin level and diastolic blood pressure at baseline and to an increase in glycosylated hemoglobin level and an increase in diastolic blood pressure from baseline to the 4-year follow-up in the younger-onset group and to higher glycosylated hemoglobin level, higher systolic blood pressure, and higher total pack-years of cigarettes smoked at baseline and an increase in systolic blood pressure from baseline to the 4-year follow-up in the older-onset groups. These data suggest that modification of three factors--hyperglycemia, high blood pressure, and smoking--may lead to a reduction in the long-term incidence of proteinuria.