Purpose: The aim of this prospective, phase II trial was to determine the response of muscle-invasive bladder cancer (MIBC) to concurrent chemoradiotherapy of weekly gemcitabine with 4 weeks of radiotherapy (RT; GemX). Patients and Methods: Fifty patients with transitional cell carcinoma, stage T2-3, N0, M0 after transurethral resection and magnetic resonance imaging, were recruited. Gemcitabine was given intravenously at 100 mg/m2 on days 1, 8, 15, and 22 of a 28-day RT schedule that delivered 52.5 Gy in 20 fractions. Chemotherapy was stopped for Radiation Therapy Oncology Group (RTOG) grade 3 bladder or bowel toxicity. The primary end points were tumor response, toxicity, and survival. Results: All patients completed RT; 46 tolerated all four cycles of gemcitabine. Two patients stopped after two cycles, and two stopped after three cycles, because of bowel toxicity. Forty-seven patients had a post-treatment cystoscopy; 44 (88%) achieved a complete endoscopic response. At a median follow-up of 36 months (range, 15 to 62 months), 36 patients were alive, and 32 of these had a functional and intact bladder. Fourteen patients died; seven died as a result of metastatic MIBC, five died as a result of intercurrent disease, and two died as a result of treatment-associated deaths. Four patients underwent cystectomy; three because of recurrent disease and one because of toxicity. One patient required a bowel resection for late toxicity. By using Kaplan-Meier analyses, 3-year cancer-specific survival was 82%, and overall survival was 75%. Conclusion: Concurrent gemcitabine-based chemoradiotherapy (ie, GemX) produces a high response rate in MIBC and has durable local control and acceptable toxicity, which allows patients to preserve their own bladder. This treatment modality warrants additional investigation in a phase III setting.