Many patients with acid-peptic disease have idiopathic gastric acid hypersecretion defined as a basal acid output > 10.0 meq/hr; however, a significant proportion have basal acid outputs > 15.0 meq/hr, which is within the range found in Zollinger-Ellison syndrome. Although idiopathic gastric acid hypersecretion is more common than Zollinger-Ellison syndrome, it is important that these two disorders be differentiated because of differences in treatment and natural history. In the present study, we compared 124 patients with idiopathic gastric acid hypersecretion and 137 patients with Zollinger-Ellison syndrome. There were no significant differences with regard to age at diagnosis, history of upper gastrointestinal hemorrhage, nausea, vomiting, and family history of duodenal ulcer and other acid-peptic disease. However, significant differences were observed between patients with idiopathic gastric acid hypersecretion and patients with Zollinger-Ellison syndrome with regard to percentage of males: 77% compared to 64% (P = 0.008), mean serum gastrin: 60 pg/ml compared to 3679 pg/ml (normal < 100 pg/ml) (P < 0.001), mean basal acid output: 15.4 meq/hr compared to 47.0 meq/hr (P < 0.001), mean age at onset of symptoms: 33 years compared to 41 years (P < 0.001), mean duration of symptoms before diagnosis: 11 years compared to five years (P < 0.001), percentage with abdominal pain: 67% compared to 82% (P = 0.00004), percentage with diarrhea: 12% compared to 75% (P < 0.000001), percentage with pyrosis: 58% compared to 40% (P = 0.003), percentage with duodenal ulcer: 53% compared to 74% (P < 0.000001), and percentage with esophagitis: 31% compared to 42% (P = 0.0004).(ABSTRACT TRUNCATED AT 250 WORDS)