PURPOSE: Preoperative chemoradiotherapy is considered standard treatment for locally advanced rectal cancer, although the scientific evidence for the chemotherapy addition is limited. This trial investigated whether chemotherapy as part of a multidisciplinary treatment approach would improve downstaging, survival, and relapse rate. PATIENTS AND METHODS: The randomized study included 207 patients with locally nonresectable T4 primary rectal carcinoma or local recurrence from rectal carcinoma in the period 1996 to 2003. The patients received either chemotherapy (fluorouracil/leucovorin) administered concurrently with radiotherapy (50 Gy) and adjuvant for 16 weeks after surgery (CRT group, n = 98) or radiotherapy alone (50 Gy; RT group, n = 109). RESULTS: The two groups were well balanced according to pretreatment characteristics. An R0 resection was performed in 82 patients (84%) in the CRT group and in 74 patients (68%) in the RT group (P = .009). Pathologic complete response was seen in 16% and 7%, respectively. After an R0 + R1 resection, local recurrence was found in 5% and 7%, and distant metastases in 26% and 39%, respectively. Local control (82% v 67% at 5 years; log-rank P = .03), time to treatment failure (63% v 44%; P = .003), cancer-specific survival (72% v 55%; P = .02), and overall survival (66% v 53%; P = .09) all favored the CRT group. Grade 3 or 4 toxicity, mainly GI, was seen in 28 (29%) of 98 and six (6%) of 109, respectively (P = .001). There was no difference in late toxicity. CONCLUSION: CRT improved local control, time to treatment failure, and cancer-specific survival compared with RT alone in patients with nonresectable rectal cancer. The treatments were well tolerated.