BACKGROUND Breath tests using stable isotopes of carbon or hydrogen are increasingly becoming established for the evaluation of various gastrointestinal functions, including measurement of exocrine pancreatic insufficiency. We wanted to evaluate the clinical relevance of the non-invasive, non-radioactive 13C-mixed triglyceride breath test in comparison with the secretin-caerulein test as the 'gold standard' of pancreatic function testing and with faecal chymotrypsin and elastase 1 in patients with mild and severe exocrine pancreatic insufficiency. METHODS The secretin-caerulein test, faecal fat analysis, 13C-mixed triglyceride breath test, faecal elastase 1, and chymotrypsin and various morphologic investigations were done in 26 patients with mild (n = 13) or severe (n = 13) exocrine pancreatic insufficiency and 25 patients with gastrointestinal diseases of non-pancreatic origin. Twenty-seven healthy volunteers served as normal controls. After a 12-h fast 200 mg mixed triglyceride (1,3-distearyl,2(carboxyl-13C)octanoyl glycerol) were orally administered with a test meal, and breath samples were taken before and at 30-min intervals for 5 h thereafter, and the increase in 13C/12C isotopic ratio in breath was analysed by mass spectrometry. Various modifications of the test procedure were investigated. RESULTS Specificity for impaired pancreatic function was higher for faecal elastase (90%) and equal for faecal chymotrypsin (82%) as compared with the various variables of the 13C-mixed triglyceride breath test (69-85%). The sensitivity of the 13C-mixed triglyceride breath test for total and separately for mild and severe exocrine pancreatic insufficiency was higher (total, 69-81%) than that of faecal chymotrypsin (total, 56%) but lower than faecal elastase (total, 92%). CONCLUSIONS The 13C-mixed triglyceride breath test very sensitively reflects severe exocrine pancreatic insufficiency (steatorrhoea) but has limited sensitivity for the detection of mild cases. With regard to the higher sensitivity and specificity, the higher practicability, and the much lower cost, determination of faecal elastase 1 concentrations is superior to the 13C-mixed triglyceride breath test and therefore remains the most reliable indirect pancreatic function test available today.