We report here on a case of mucinous adenocarcinoma that probably originated in the renal pelvis of a horseshoe kidney. A 61-year-old woman presented with a palpable mass in the left upper quadrant of the abdomen, and this mass had been present for several months. Computed tomography (CT) revealed a left renal pelvic tumor in the horseshoe kidney. Grossly, a 10x9x8 cm unilocular cystic mass filled with chocolate colored mucinous fluid was seen. A connection between the cystic mass and the renal pelvis was demonstrated on retrograde pyelography. Microscopically, the cyst contained anaplastic columnar mucosecretory epithelial cells. Some atypical cell clusters were freely floating in the mucinous lakes. The histopathological findings were consistent with mucinous adenocarcinoma. In addition, glandular metaplasia was noted in the cystic wall. Immunohistochemical assessment of the pelvic adenocarcinoma revealed the positive expressions of carcinoembryonic antigen (CEA) and cytokeratin 20 (CK20) and a weak positive expression of cytokeratin 7 (CK7).