While the association between pancreatic cancer and diabetes is well recognized, little is known about glucose tolerance and insulin secretion in patients with this tumour. Thirty patients with pancreatic cancer not complicated by diabetes, 10 with nonpancreatic cancer, and 10 healthy subjects were studied for glucose tolerance and insulin secretion in response to an oral glucose load. Twenty of the 30 patients with pancreatic cancer (70%) had impaired glucose tolerance compared with none of the patients in the other two groups. In most of these 20 patients' insulin responses were higher than those of patients with non-pancreatic cancer or healthy subjects. The results indicate that about two-thirds of patients with pancreatic cancer have abnormal glucose tolerance; this intolerance is associated with elevated insulin secretion, suggesting that it may be due to a peripheral resistance to insulin.