Systemic mastocytosis is a rare disorder that infrequently affects the GI tract. Bowel involvement in mastocytosis is characterized by thickened folds and small mucosal nodules, and there is an increased incidence of peptic ulcer disease and malabsorption. This paper describes a new case of mastocytosis that presented radiographically as 1.0-1.5 cm gastric and duodenal nodules. Some of the duodenal nodules were bull's-eye lesions with central collections of barium. Mastocytosis, along with primary neoplasms, aberrant pancreas, eosinophilic granuloma, and metastases should be included in the differential for bull's-eye lesions of the GI tract.