RESUMEN
CASE DESCRIPTION: A nearly 6-year-old female spayed Labrador Retriever was presented for acute abdominal pain and lethargy. The dog had no previous health concerns apart from occasional episodes of urinary incontinence in the 2 months prior to presentation. A retroperitoneal mass involving the right ureter was found during the investigations. Serum urea was mildly elevated, but the serum creatinine was within the normal range. No distant metastases were detected. A right ureteronephrectomy was performed. The ureteral mass was confirmed as a leiomyosarcoma and completely excised. The kidney was histologically normal. Unfortunately, during a routine 3-month postoperative assessment, a recurrent mass at the previous retroperitoneal surgical site was confirmed by biopsy to be a leiomyosarcoma. Courses of doxorubicin and chlorambucil were given, but failed to halt the progression of the recurrent mass. The dog was euthanised 5.5 months postoperatively because of poor quality of life. CLINICAL RELEVANCE: Ureteral leiomyosarcoma should be on the differential diagnosis list for a retroperitoneal mass, possibly causing severe abdominal pain with minor clinical signs associated with the urinary tract. This dog in this reported case of ureteral leiomyosarcoma had a short survival time, despite complete surgical excision and chemotherapy, because of local recurrence.