Background: Transitional cell carcinoma (TCC) of the urinary bladder of dogs can be a difficult cancer to treat, and effective therapies are limited. Vinblastine has been used in humans with TCC and has potent anti-proliferative effects against canine TCC cells in vitro. Objectives: To determine the antitumor activity and toxicoses of vinblastine in dogs with urinary bladder TCC. Animals: Animals selected were 28 privately owned dogs that presented to the Purdue University Veterinary Teaching Hospital (PUVTH) with measurable, histologically confirmed TCC. Methods: Prospective clinical trial: The starting vinblastine dosage was 3.0 mg/m 2 IV every 2 weeks. Treatment continued until cancer progression or unacceptable toxicoses occurred. Complete evaluations (physical exam, complete blood count [CBC], serum biochemical profile, urinalysis, thoracic radiography, abdominal ultrasound [US]) were performed at 8-week intervals. Urinary tract US with bladder tumor mapping was performed monthly. Toxicoses were graded according to Veterinary Co-Operative Oncology Group (VCOG) criteria. Results: Tumor responses included 10 (36%) partial remission, 14 (50%) stable disease, and 4 (14%) progressive disease. The median progression free interval was 122 days (range, 28-399 days). The median survival time was 147 days (range, 28-476 days) from 1st vinblastine treatment to death and 299 days (range, 43-921 days) from diagnosis to death. The majority of dogs (27 of 28) did not have clinically relevant adverse effects. Seventeen of 28 (61%) dogs required dosage reductions because of neutropenia. Conclusion and Clinical Importance: Vinblastine has antitumor activity against TCC in dogs and can be considered another treatment option for this cancer.