The prevalence of gastric carcinoid in fundic atrophic gastritis is probably greater than previously recognized. To help elucidate the clinicopathology of this syndrome, we report a series of 11 patients with solitary or multicentric carcinoid tumors. In these patients, basal gastrin levels and density of fundic mucosal endocrine cells were greater than that for patients with uncomplicated fundic atrophic gastritis (p = 0.02 and p = 0.002, respectively). The polypoid tumors, of which the largest measured 30 mm, frequently showed characteristic endoscopic features. They were all situated in the fundic mucosa, which showed micronodular endocrine cell hyperplasia. Small, endoscopically evident tumorlets, or "early carcinoids," limited to the lamina propria were observed in some patients. These lesions may represent intermediate stages between micronodules and invasive carcinoids, all of which infiltrated at least into the muscularis mucosae of the gastric wall. Although some consistent characteristics features were noted, there were structural variations. The cells were argyrophil but nonargentaffin and did not stain with conventional mucus stains. They did not stain significantly for carcinoembryonic antigen (CEA). The secretory product of these tumors remains to be identified. Ultrastructurally, some tumors were mainly composed of enterochromaffinlike (ECL) cells, but in other tumors most of the cells could not be classified.