The prevalence of pancreatic exocrine insufficiency (PEI) in diabetes mellitus (DM) varies widely between studies, which may be explained by methodological problems. We aimed to establish the prevalence of PEI in DM using the faecal elastase-1 (FE-1) assay as a screening test, and to further investigate these patients by the mixed 13C-triglyceride (13C-MTG) breath test.