In insulin dependent diabetics microalbuminuria predicts proteinuria which is associated with an extremely high relative mortality rate. We studied the connection between long term blood glucose levels and microalbuminuria. One hundred and twenty-seven patients between 10-20 yr of age were screened for microalbuminuria. Twenty patients with both persistent microalbuminuria (greater than 15 micrograms/min albumin in at least two out of three timed overnight urine samples) and greater than 4 measurements of glycosylated hemoglobin A1 yearly for 5 yr, were included in the study. These 20 patients were matched with respect to sex, age and duration of diabetes against the normoalbuminuric diabetics. The patients with microalbuminuria had significantly higher mean 5-yr glycosylated hemoglobin A1 values than those with normoalbuminuria, 12.4 and 10.5% respectively (p less than 0.01). The data indicate a logarithmic association between mean 5 yr glycosylated hemoglobin A1 values and the urinary albumin excretion rate. The study points to a strong association between long term blood glucose levels and microalbuminuria in adolescents.