A patient with pre-existing diabetes mellitus who was receiving corticosteroids in an effort to prevent transplant rejection was presented. The patient's hyperglycemia was exacerbated by the corticosteroids, and the discussion centered around this aspect of the case. The clinical effects of glucocorticoids on carbohydrate metabolism were reviewed, and four general points were made: (1) the chronic effects of corticoids on glucose tolerance are much less pronounced than the acute effects; (2) the degree of impariment is proportional to the pre-existing status of glucose tolerance; (3) development of frank diabetes mellitus in a previously normal patient is unusual; and (4) the abnormalities of carbohydrate metabolism induced by glucocorticoids fit the pattern of an insulin resistant state. The subject was reviewed and relevant in vivo and in vitro observations were presented in support of the above concepts, and to help identify the pathophysiologic mechanisms involved. It was postulated that glucocorticoids affect glucose metabolism by decreasing glucose utilization and by increasing hepatic glucose production. The possible mechanisms underlying these effects were discussed.