A developmentally normal 6-year-old boy was admitted to our hospital for evaluation of lower abdominal pain on the right side. Physical examination revealed a hard and well-demarcated mass. Routine laboratory tests were normal. Subsequently, an abdominal CT was performed using a GE LightSpeed scanner (GE Medical Systems, Milwaukee, WI, USA), with parameters of 120 kV and 180 mA. This revealed a mildly lobulated, well-defined homogeneous soft-tissue mass, 5.0 × 5.7 × 6.7 cm in size, in the right lower abdominal cavity, and there was no evidence of pelvic lymphadenopathy (Figure  1). The lesion had a density of 41 HU (Hounsfield units); it had no hemorrhagic, necrotic, or cystic components. Following infusion of a contrast agent, the tumor demonstrated marked homogeneous enhancement, and a density of 81 HU. Laparotomy revealed a tan-colored mass arising from the mesoappendix, without adhesions to the appendix or other organs and structures (Figure  2). Macroscopically, the mass was well-circumscribed, with an incomplete capsule, and all margins were negative. Microscopically, the tumor consisted of proliferating spindle cells and epithelioid cells. Mitotic figures were noted in 12 of 50 high-power fields. Immunohistochemical staining revealed that the tumor was diffusely and strongly positive for c-Kit (CD117) (Figure  3), myeloid stem cell antigen (CD34), DOG1, and Ki-67, slightly positive for vimentin, and negative for smooth muscle actin (SMA), neuron-specific enolase (NSE), S-100, and desmin. Based on these morphological and immunohistochemical findings, the final pathological diagnosis was that of a malignant gastrointestinal stromal tumor of the mesoappendix. The patient was treated by administration of Glevec as an adjuvant postoperative chemotherapy and has been living disease-free for 9 months of follow-up.