OBJECTIVE A prospective phase II trial was conducted to evaluate the feasibility, safety, and pathologic response rate of preoperative capecitabine and accelerated synchronous integrated boost (SIB) intensity-modulated radiotherapy (IMRT) in patients with locally advanced rectal cancer. METHODS Consenting operable patients with stage II or III adenocarcinoma of the rectum received capecitabine (825 mg/m2 PO BID, 5 days/wk x 5 weeks) and SIB-IMRT delivering 55 Gy (2.2 Gy/fraction) to the gross tumor while simultaneously delivering 45 Gy (1.8 Gy/fraction) to the regional lymph nodes and areas at risk for harboring microscopic disease. Total mesorectal excision followed 6 weeks later. A single pathologist analyzed the resected tumor's TNM stage and Mandard regression/response scores. The primary end point was pathologic complete response (pCR) rate. RESULTS Ten subjects were enrolled, 2 of which were ineligible (1 screening failure and 1 unrelated cerebrovascular accident occurring early in treatment). The remaining 8 patients were evaluable. All 8 completed chemoradiation with strict compliance to the protocol schedule and then went on to surgical resection. At a median follow-up of 26 months (range, 15-40), all patients were alive without evidence of recurrent disease. The crude pCR rate was 38% with 50% achieving down-staging. Of 3 patients who had tumors within 5 cm of the anal verge, 2 underwent sphincter-sparing procedures. Grade 4 diarrhea occurred in 1 of 8 (13%) patients. The remaining toxicities were grade 1 or 2. CONCLUSIONS Preoperative chemoradiation with capecitabine and SIB-IMRT is well tolerated and results in an encouraging pCR rate for patients with locally advanced rectal cancer.