The University Group Diabetes Program (UGDP), a long-term prospective clinical trial, was designed to evaluate the effects of various hypoglycemic agents on vascular complications in patients with asymptomatic, adult-onset diabetes. This study provided no evidence that phenformin was more efficacious than diet alone or than diet and insulin in prolonging life for the patients studied. In fact, the observed mortality from all causes and from cardiovascular causes for patients in the phenformin treatment group was higher than that observed in any of the other treatment groups. In addition, there was no evidence that phenformin was more effective than any of the other treatments in preventing the occurrence of nonfatal vascular complications associated with diabetes. For these reasons, the use of phenformin has been terminated in the UGDP. Data collection in this study is continuing for all surviving patients. The additional information obtained over a longer period of follow-up on patients in the two insulin groups compared with the patients in the placebo group will provide a more adequate basis for the assessment of the long-term effects of insulin.