Diabetes in patients with pancreatic cancer occurs in 70% to 80% of the patients and is characterized by high plasma levels of insulin. In type II diabetes that is not associated with pancreatic cancer, peripheral insulin resistance and impaired muscle glycogen synthesis are major pathogenic factors. We investigated peripheral insulin sensitivity in patients with pancreatic cancer before and after tumor removal. The effects of pancreatic tumor extracts on glycogen synthesis in skeletal muscle in vitro and the tumor content of pancreatic islet hormones were also investigated. Marked peripheral insulin resistance was found in the patients with pancreatic cancer and was more pronounced in the diabetic patients than in the nondiabetic patients. Insulin sensitivity was not correlated with weight loss, tumor size, or bilirubin levels but improved after surgery. Tumor extracts from diabetic patients with pancreatic cancer caused a marked reduction of glycogen synthesis in skeletal muscle in vitro. All tumors contained islet hormones but not in concentrations sufficient to explain the effect on glycogen synthesis. These findings indicate that a diabetogenic factor associated with pancreatic adenocarcinomas could be involved in the development of the profound peripheral insulin resistance and thereby could contribute to the high incidence of diabetes observed in patients with pancreatic cancer.