Hyperglycemia and glucose intolerance are characteristic of both the metabolic responses to injury and normal aging. To evaluate the interaction of such changes we conducted hyperglycemic glucose clamp studies (2-hour) in previously healthy young (20 +/- 4 years, Mean +/- SD) and older (73 +/- 9 years) trauma patients and volunteers (23 +/- 1 and 68 +/- 5 years), determining whole-body glucose disposal and insulin responses to hyperglycemia. Injury Severity Scores were similar in the young and older patient groups (range, 17-30). Plasma glucose levels were greater in patients than volunteers in both the basal and hyperglycemic periods. Basal serum insulin and C-peptide levels were similar among groups, but during hyperglycemia both were markedly higher in young patients than older (and both volunteer groups). Whole-body disposal of exogenous glucose was substantially lower in patients than volunteers and in the older groups. Aging has a major impact on postinjury metabolism, being associated with exaggerated glucose intolerance and diminished insulin responses to glucose infusion. These findings have important implications for the metabolic and nutritional care of older patients following trauma and during critical illness.