Adult patients with cystic fibrosis (CF) are at high risk for developing insulin-dependent diabetes mellitus. Therefore, the fast insulin release (FIR) to intravenously administered glucose was measured in 23 adult CF patients. The influence of the clinical parameters and type of gene deletion on the amplitude of the FIR, defined by the sum of the 1st- and 3rd-minute insulin concentrations was analyzed. In 11 of the 18 normoglycemic patients with exocrine pancreatic insufficiency and the 3 nontreated diabetic CF patients studied, an FIR value lower than the 3rd percentile was found. The female patients had higher mean FIR values than the male patients (62.8 ± 39.6 vs. 27.9 ± 17.9 mU/l; p < 0.05). No influence of age, body mass index, or pulmonary or liver involvement on the FIR was found. Subjects heterozygous for the delta F508 deletion had a similar insulin response as homozygous patients. The FIR level correlated negatively with the basal glucose level (r = 0.4; p < 0.001). In conclusion, 61 % of the adult nondiabetic CF patients with exocrine pancreatic insufficiency presented a loss in acute insulin response, which could not be predicted by clinical or genetic parameters.