Gastrointestinal stromal tumors (GIST) represent the most usual mesenchymal tumors of the gastrointestinal tract. In this paper we present a case of a 59-year old patient with an adenocarcinoma of the transversal colon near the lienal flexure without breaking of the intestinal membrane synchronous with an intramural tumor in the antral region of the stomach which was not registered before the operation. Final immunohistochemical findings of the operation specimens: 1. Adenocarcinoma of the large intestine (histological grade I, nuclear grade I, Dukes A, invasion to submucosa) and 2. GIST with low malignant potential. Synchronous appearance of GISTs and colorectal adenocarcinoma, although very rare, has been increasingly observed in the last five years. Because of the small number of cases, a causal connection between these different types of tumors cannot be proven. Through this case report we hope to contribute to the adequate preoperative preparation, operation technique and postoperative monitoring of such patients. Surgical treatment is a dominant way of treating these synchronous tumors, so the surgeon has high responsibility for adequate selection of operative procedure on these patients.