To the Editor: The age distribution, according to type of diabetes, of end-stage renal disease in Michigan reported by Cowie et al. (Oct. 19 issue)1 sheds light on problems with the classification of diabetes as juvenile or adult-onset in preliminary 1988 Medicare data on end-stage renal disease.2 In these Medicare data, the fact that a large number of cases of diabetic end-stage renal disease in persons over 55 years old are attributed to juvenile diabetes suggests the presence of misclassification (probably because of insulin treatment). Thus, the 56.8 percent of cases reported by Medicare that are attributed to adult-onset diabetes. © 1990, Massachusetts Medical Society. All rights reserved.