The hypothesis that fecal fat concentration (FFC; g fat/100 g wet stool weight) should be higher in pancreatic steatorrhea than in steatorrhea due to gastrointestinal disease was investigated. Fecal weight and fat excretion were measured in 59 patients with pancreatic steatorrhea and in 53 patients with nonpancreatic steatorrhea. A further 42 patients were studied who had normal fecal fat excretion but exocrine pancreatic insufficiency proven by a secretin-pancreozymin test along with 31 patients with well-defined gastrointestinal disease but without steatorrhea. The control group consisted of 184 subjects without evidence for malabsorption. FFC was significantly higher in patients with pancreatic compared to those with nonpancreatic steatorrhea. However, because of the almost identical concentrations in patients with steatorrhea due to celiac sprue (FFC 10.5 +/- 4.3%, mean +/- SD) and those with pancreatic steatorrhea (FFC 11.5 +/- 4.7%), FFC was of no value for the differential diagnosis between pancreatic and nonpancreatic steatorrhea.