In 34 patients suspected of having pancreatic cancer glucose homeostasis and insulin secretion in response to intravenous glucagon (1 mg) were studied prospectively. Twelve patients had pancreatic cancer at laparotomy and the remainder were designated a control group. Responses were also measured in 12 healthy volunteers. There was no significant difference in the rise of blood glucose between the groups after glucagon. The mean plasma insulin concentrations rose rapidly in both groups after glucagon but were significantly lower in the pancreatic cancer group. In patients with obstructive jaundice the plasma insulin response was a better discriminator of pancreatic cancer. Abnormal pancreatic beta-cell function is detectable in patients with pancreatic carcinoma before any change in glucose homeostasis, particularly in patients with obstructive jaundice. The glucagon stimulation test may be useful in the earlier diagnosis of pancreatic carcinoma before the development of clinically overt diabetes mellitus.