ABSTRACT. The relationship between diabetes mellitus and the exocrine pancreatic function was evaluated in 29 patients with insulin‐dependent diabetes mellitus by measuring cathodic trypsin‐like immunoreactivity (TLI) and the enzymatic activity of pancreatic isoamylase in serum before and 90 min after breakfast and insulin. Thirty healthy subjects served as reference group. Median fasting serum concentrations in the diabetic subjects were significantly lower than in the reference subjects: for TLI 143 and 299 μg trypsin standard/I and for pancreatic isoamylase 43 and 101 U/I, respectively (p<0.001 for both). A positive correlation between TLI and pancreatic isoamylase was present in the diabetics (Sperman's rho=0.56, p<0.01). The serum concentrations of TLI and pancreatic isoamylase were not related to the duration of diabetes, daily insulin dose or glycemic control measured by blood glucose and total glycosylated hemoglobins. Cathodic TLI and pancreatic isoamylase in serum were not influenced by food and insulin.