A synthetic "mixed" triglyceride (1,3-distearoyl,2[13C]octanoyl glycerol), having a medium-chain fatty acid in the 2 position, was evaluated as a substrate for an exocrine pancreatic function test by comparing the 13CO2 breath excretion with the lipase output in the duodenum in 25 normal subjects, 29 patients with pancreatic disease, and 22 patients with steatorrhea of nonpancreatic origin. Excellent correlation was found in normal subjects and patients with pancreatic disease (r = 0.89) between lipase output in the duodenum and the 6-h cumulative 13CO2 excretion in breath, indicating that the mixed triglyceride breath test is an excellent noninvasive test of pancreatic lipase activity in the duodenum. As a test of exocrine pancreatic insufficiency, it has a sensitivity of 0.89 and a specificity of 0.81.