Chromogranin A (Cg A) is a protein that is coreleased with peptide hormones from gut endocrine cells and tumors. Plasma levels of Cg A, pepsinogen group I, and gastrin were measured in 31 patients with gastrinoma. Mean Cg A level in 10 patients with gastrinoma who were not operated on was 169 +/- 32 ng/mL, while in 9 control patients it was 28 +/- 5 ng/mL. In 18 patients with gastrinoma with residual tumor after total gastrectomy, the mean Cg A level was 45 +/- 6 ng/mL, and in 10 patients with normal gastrin levels after total gastrectomy and tumor excision, the mean Cg A level was 40 +/- 4 ng/mL. In 7 patients in whom pregastrectomy and postgastrectomy Cg A levels were measured, the mean reduction was 94 +/- 27 ng/mL, or 66%. There was no correlation between Cg A levels and amount of tumor, presence of metastases, or multiple endocrine neoplasia type I syndrome. There was a significant correlation between Cg A and pepsinogen I levels but no correlation between Cg A and gastrin levels. The results suggest that the elevated plasma Cg A levels in patients with gastrinoma are determined primarily by the trophic effects of gastrin on gastric enterochromaffinlike cells rather than by corelease from the gastrin-producing tumor itself.