Hypergastrinemia is a very important clinical condition for the reason that a growing body of evidence obtained from animal and human experiments has revealed gastric carcinoids induced by hypergastrinemia. We investigated 35 patients with Basedow's disease (BD) to elucidate the mechanism of hypergastrinemia associated with BD as well as the relationship between type A gastritis and BD. Fasting serum gastrin levels in BD (296.1 +/- 251.4 pg/ml; mean +/- S.D.) were significantly (p less than 0.001) higher than those in age-matched 27 healthy subjects (106.1 +/- 69.2), and in the BD group, significant positive correlation was detected between fasting serum gastrin levels and thyroid hormones (i.e. T3 and free T4). In the hyperchlorhydria group in BD with hypergastrinemia, the levels of fasting serum gastrin were normalized after euthyroidism was attained due to antithyroidal drugs. On the other hand, in the achlorhydria group in BD significant hypergastrinemia was persisted in spite of normalization of thyroid function. Twenty % of the BD patients had histologically proved type A gastritis with achlorhydria, and all patients with type A gastritis were older than 60 years old. Endoscopic examination revealed that one patient with type A gastritis had an early gastric cancer. However, no gastric carcinoids were demonstrated in this study. In conclusion, the results described as above suggested, 1) hypergastrinemia observed in patients with BD may be induced by gastrin hypersecretion due to hyperthyroidism as well as type A gastritis, 2) BD patients with type A gastritis were recommended to undertake regular endoscopic examination for detecting gastric cancers as well as gastric carcinoids.