Exocrine pancreatic function was studied in patients with long-standing insulin-dependent diabetes mellitus using the secretin-pancreozymin test (n = 53), and estimation of immunoreactive trypsin (n = 43) and pancreatic isoamylase (n = 43). The secretin-pancreozymin test was abnormal in 23 patients (43%). The abnormalities found were a decreased output of lipase (37%), amylase (36%) or trypsin (26%) and bicarbonate (15%). Serum immunoreactive trypsin was below normal in only 6 (14%) and pancreatic isoamylase in 29 (67%) patients. There was no correlation between impairment of the secretin-pancreozymin test and decreased serum enzyme levels. It is concluded that an impairment of exocrine pancreatic function is frequent in insulin-dependent diabetics but that a decrease in serum enzymes, especially in pancreatic isoamylase, does not reflect an impairment of pancreatic function in these patients.