Fecal chymotrypsin (CT) activities were determined in 149 randomly collected fecal specimens of 80 patients in whom the pancreatic function had been tested by a secretin-cholecystokinin test. There was a significant correlation between fecal CT activities and outputs of trypsin (r = 0.3451, p less than 0.01) and amylase (r = 0.3285, p less than 0.01) in duodenal juice. Fecal CT activities were normal in all patients who--based upon enzyme outputs in duodenal juice after stimulation with secretin and CCK/PZ--were classified as 'borderline cases', in most patients with 'low-normal' pancreatic function, and in a significant number of patients with established insufficiency of exocrine pancreas. On the other hand, fecal CT activities were abnormal in patients with severely impaired output of trypsin in duodenal juice, and only 7% of the fecal specimens from patients with established normal function of exocrine pancreas had abnormal low CT activities. It is concluded that the sensitivity of the fecal enzyme method is rather low as compared to the secretin-cholecystokinin test, but that fecal CT determinations give valuable diagnostic information in patients with more pronounced insufficiency of the exocrine pancreas.