Diabetes included several disorders associated with hyperglycemia. A difference in inheritance between the families of juvenile-onset- and maturity-onset-type diabetics, provides evidence for genetic heterogeneity. Heterogeneity of insulin responses to glucose was foung among nonobese patients with maturity-onset-type diabetes. Prospective studies in young patients have shown that glucose intolerance may not progress for as long as 22 years and that subnormal insulin responses to glucose have not decreased further, up to 12 years. However, patients who progressed to diabetes requiring insulin had insulin responses that were subnormal or below the control mean. None whose insulin responses exceeded this mean have decompensated. Thus, insulin response to glucose has prognostic implications. A tentative classification of diabetes in the young is proposed. There was a significant correlation between muscle capillary basement membrane width and known duration of carbohydrate intolerance.