A 83-year old man treated with naproxen during two years was admitted because of hypovolemia and peritoneal irritation. A panendoscopic study was performed and an ulcer localized at the large curvature of the stomach was disclosed. In the gastrectomy specimen the ulcer showed necrosis, edema, fibrosis, chronic inflammatory infiltrate with lymphocytes and plasma cells. Additionally, atypical cells with irregular and hyperchromatic nuclei or vacuolated cytoplasm were seen in the lamina propia and infiltrating the muscular layers; isolated signet-ring-like cells were also seen. Histochemical study with periodic acid-Schiff, mucicarmin, and colloidal stains revealed mucosubstances in these cells. A poorly differentiated carcinoma was initially diagnosed. However, the immunohistochemical study were positive for histiocytic markers (CD-68, S-100 protein) and negative for epithelial markers (cytokeratin; and epithelial membrane antigen). The positivity of mucus stains in the histiocytes could be explained in this case by phagocytosis of mucous substances released from broken hyperplastic glands in the vicinity of the ulcer. To our knowledge, atypical histiocytic infiltration in gastric ulcers has not been previously described; thus, it should be included in the group of gastric carcinoma mimicks.