Several of the characteristic complications of diabetes mellitus resemble age-like changes in collagen-rich tissues. It has been reported that increased glucosylation of hemoglobin and serum proteins occurs in diabetes. Glucosylation of insoluble human tendon collagen, a protein with little or no turnover was determined by a thiobarbituric acid method in 23 subjects as a function of age and the presence or absence of diabetes. Amounts of glucose and collagen solubilized by collagenase digestion of the samples were also determined. Glucosylation of collagen was found to increase with age and was markedly increased in juvenile onset diabetes. There appeared to be a limit to the amount of glucosylation that could occur, and older individuals with maturity-onset diabetes demonstrated glucosylation within that limit. The glucose nonenzymatically bound to human collagen may indicate the level of long-term control of the diabetes, and may play a role in the alteration of collagenous tissue properties that occurs in both aging and diabetes.