In 12 patients with biopsy-proven pancreatic ductal adenocarcinoma, the following were determined: (1) whether decreased food intake, malabsorption, or altered fat metabolism were associated with weight loss; (2) the effect of pancreatic extract as treatment for malabsorption; and (3) the accuracy of the triolein breath test for detection of steatorrhea. Weight loss occurred in 11 patients and only in patients who had either malabsorption (n = 5), low coefficients of caloric consumption (n = 2), or both (n = 4). Nine patients had fat malabsorption, six had protein malabsorption, and caloric consumption was decreased in seven patients. Metabolism of oleic acid was significantly decreased (P less than 0.01) compared to normal subjects and correlated with basal metabolic rates (r = 0.6; P less than 0.05) which were within the range of normal values for age and sex. Body weight loss correlated only with coefficients of fat and protein absorption (r = 0.59; P less than 0.05). Treatment of patients with pancreatic extract resulted in significant improvement in absorption in those with moderate to severe fat or protein malabsorption (coefficient of absorption less than 80%) but no significant improvement occurred in patients with mild fat or protein malabsorption. The triolein breath test was abnormal in all patients with fat malabsorption and predicted improvement of fat absorption in five of six patients with steatorrhea who were treated with pancreatic extract. Thus, in pancreatic cancer, weight loss is associated with malabsorption; exogenous pancreatic extract significantly improves moderate to severe fat or protein malabsorption, and the triolein breath test detects fat malabsorption and predicts the treatment response to pancreatic extract.