Concurrence of adenocarcinoma and neuroendocrine carcinoma in the gastrointestinal tract has rarely been observed. We report two cases of gastric collision tumors (adenocarcinoma and neuroendocrine carcinoma) that developed in a 64-year-old man and a 71-year-old man. In both cases, there was a single ulcerative lesion in the stomach. Histologically, the gastric lesions were composed of two discrete lesions: tubular adenocarcinoma at the edge of an ulcer and neuroendocrine carcinoma in the ulcer base. We will discuss collision and composite tumors.