Desmoid tumors are rare benign tumors with aggressive fibroblastic proliferation. Although desmoid tumors do not metastasize, they have locally aggressive features and can cause a urinary fistula. Here, we report a case of a 35-year-old woman with Gardner syndrome who was diagnosed with an intra-abdominal desmoid tumor 1 year previously and who presented with a newly developed cystic mass lesion on a computed tomography scan. The cystic mass lesion was clinically diagnosed as an urinoma from the right ureterotumoral fistula; thus, surgical resection of the mass lesion was planned. However, Tc-99m diethylenetriamine pentaacetic acid renal scintigraphy revealed bilateral ureterotumoral fistulas; hence, the treatment plan was changed to conservative management.