Increased basal and/or pentagastrin-stimulated gastric acid secretion was observed in 31 of 40 cystic fibrosis (CF) patients as compared to 13 age-matched control volunteers. Fasting serum gastrin level for 20 CF patients of 67.7 +/- 11.1 pg/ml (ranging from 10 to 145 pg/ml) was normal. Meal-stimulated serum gastrin concentrations were not significantly higher for seven CF patients as compared to nine normal subjects. In a rat bioassay, serum extracts from six CF patients produced gastric acid secretion comparable to 250 ng/ml of pentagastrin; significantly (p less than 0.01) greater than the responses produced by serum extracts from six normal subjects. There was no significant correlation of severity of pulmonary disease or steatorrhea with acid secretory results. Fourteen of 16 CF patients biopsied had evidence of proximal small intestinal injury which correlated with basal acid output elevation. Gastric acid hypersecretion in CF patients is due to nongastrin secretagogues and is a cause of proximal small intestinal injury.