Case Description: A 14.5 year old male castrated West Highland White Terrier was evaluated for a brief history of pollakiuria and suspected urinary tract infection. Clinical Findings: Abdominal ultrasonography revealed a dorsal bladder wall mass that exhibited measurable growth over one month's time. Urinalysis, urine cytology, and a Bladder Tumor Antigen test failed to provide a diagnosis for this mass. An abdominal celiotomy with partial cystectomy was performed and histopathology results revealed a Transitional cell carcinoma. Treatment and Outcome: The patient recovered well following surgery and was treated with piroxicam and misoprostol. Treatment with mitoxantrone was recommended but declined at that time. Three months following diagnosis the patient has yet to develop lower urinary tract clinical signs or other manifestations of a malignant disease process. Clinical Relevance: Transitional cell carcinomas are the most common malignant neoplasm found in the lower urinary tract of dogs. Although many diagnostic tests are available to clinicians, biopsies may be the only capable method of providing the definitive diagnosis of Transitional cell carcinoma. Treatment of this neoplasm will vary on an individual patient basis, but may include surgery, chemotherapy, and radiation therapy.