Precise relationships between pancreatic ductal obstruction and pancreatic secretory capacity have not been established. In this study, we describe the quantitative relationships between the lengths of opacified ducts obtained at retrograde pancreatography and the secretory capcity of the gland for volume, bicarbonate, lipase, and trypsin. Forty-five patients (17 with pancreatic cancer, 6 pancreatitis, 5 other malignancies, and 17 nonmalignant, nonpancreatic disease found at laparotomy) were studied with a method of duodenal intubation and perfusion with basal saline perfusion alone or with continuous intravenous infusion of secretin or of cholecystokinin-pancreozymin. Secretory outputs of volume, bicarbonate, and enzymes compared with the length of opacified ducts showed a significant (P less than 0.05) linear relationship for patients with pancreatic cancer, pancreatitis, and other cancers. The resulting data imply that obstruction of the pancreatic duct is important in decreasing secretion of the pancreas in pancreatic disease. The relationship between obstruction and pancreatic secretion demonstrates that a decrease in exocrine pancreatic secretion cannot be detected until more than 60% of the total length of the main pancreatic duct has been obstructed.