Background: This phase I/II trial assessed the safety, feasibility and efficacy of cetuximab (Cet) in combination with 5-fluorouracil (5FU) and mitomycin C (MMC) with concurrent radiotherapy (RT) for the treatment of muscle invasive bladder cancer (MIBC). BC2001 trial has previously reported significant improvement in locoregional control in patients (pts) with MIBC who were randomised to synchronous chemo-RT compared to RT alone (James, Hussain, Hall et al NEJM 2012). Results for phase I have been reported previously. This abstract reports the combined results of phases I/II. Methods: From September 2012 to October 2016, 33 pts were recruited (7 pts to phase I from 2 UK centres and 26 patients to phase II from 5 UK centres). Pts received loading dose of Cet 400 mg/m² followed by weekly Cet 250 mg/m² for 7 weeks, continuous infusion 5FU 500mg/m²/day during fractions 1-5 and 16-20 of RT and MMC 12mg/m²on day 1 in combination with radical RT 64 Gy in 32 fractions. Neoadjuvant chemotherapy was mandatory in phase I but optional in phase II. Primary outcomes in phase I were feasibility and toxicity. Primary outcome in phase II was 3 month pathological complete response (CR) rate, secondary outcomes included toxicity, progression free survival (PFS) and overall survival (OS). Results: Median age of pts was 70 (range 46.9-85.6). Treatment completion rates in phase I were RT 100%, 5FU 100 %, MMC 100%, Cet 96%. Of the 28 analysable pts, phase II primary outcome data was available for 25 pts at the time of analysis with a 3 month pathological CR rate of 88%. 5 local progressions and 4 deaths were reported. 12 pts suffered at least one SAE. Grade 4 toxicities observed were dyspnoea, atrial fibrillation, interstitial pneumonitis, sepsis, thromboembolism, neutropenia and palpitations. The commonest grade 3 toxicities were skin rash, diarrhoea, low platelet count, low white blood cell count, fever and haematuria. The most common grade 1 and 2 toxicities were diarrhoea and skin rash. Data on PFS and OS will be presented. Conclusions: It was feasible and safe to add cetuximab to full dose chemo-RT with 5FU/MMC. The CR rate is encouraging and further randomised studies with this combination are warranted. Clinical trial information: ISRCTN80733590.