Cyclosporin is an immunosuppressive drug used with increasing frequency in patients with diabetes mellitus both as experimental primary therapy for insulin-dependent diabetes mellitus and as therapy accompanying pancreatic transplantation. However, reports have appeared contending that cyclosporin causes glucose intolerance and inhibits pancreatic islet β-cell function. Consequently, concern has been raised that the beneficial effects of immunosuppression may be offset by adverse metabolic effects of the drug. To address this issue, we examined intravenous glucose tolerance and pancreatic islet β-cell function in a group of nondiabetic multiple sclerosis patients before and during a 2-yr course of cyclosporin or placebo therapy. Patients were randomly assigned to one of the two drug groups and followed in a double-blind manner. Basal levels of glucose, insulin, and C-peptide as well as glucose disappearance rates and pancreatic islet β-cell function after stimulation with intravenous glucose and arginine were determined immediately before therapy and after 3 wk, 6 mo, 1 yr, and 2 yr of therapy. No abnormalities in these parameters were observed in the cyclosporin or the placebo-treated group. It appears that cyclosporin can be given in conventional doses for as long as 2 yr without encountering evidence for impaired glucose homeostasis. However, whether adverse effects will materialize over longer periods of drug use remains a question.