We studied whether microalbuminuria (30 to 140 micrograms of albumin per milliliter) would predict the later development of increased proteinuria and early mortality in Type II diabetics. During 1973, morning urine specimens of diabetic clinic patients 50 to 75 years of age whose disease had been diagnosed the age of 45 were examined for albumin level by radioimmunoassay. Seventy-six patients with albumin concentrations of 30 to 140 micrograms per milliliter were identified for long-term follow-up. They were compared with normal controls, diabetic patients with lower albumin concentrations (75 patients with concentrations less than 15 micrograms per milliliter and 53 with concentrations of 16 to 29 micrograms per milliliter), and 28 diabetic patients with higher concentrations (greater than 140). Age, duration of diabetes, treatment method, fasting blood glucose level, blood pressure, height, and weight were determined for the four diabetic groups. After nine years the group with albumin concentrations of 30 to 140 micrograms per milliliter was more likely to have clinically detectable proteinuria (greater than 400 micrograms per milliliter) than were the groups with lower concentrations. Mortality was 148 per cent higher in this group than in normal controls--comparable to the increase (116 per cent) in the group with heavy proteinuria (albumin levels greater than 140 micrograms per milliliter). In addition, mortality was increased 76 per cent in the group with albumin levels of 16 to 29 micrograms per milliliter and 37 per cent in the group with levels below 15. We conclude that microalbuminuria in patients with Type II diabetes is predictive of clinical proteinuria and increased mortality.