OBJECTIVE To determine the maximum tolerated dose (MTD) and recommended dose (RD) of irinotecan combined with preoperative chemoradiotherapy with S-1 in patients with locally advanced rectal cancer. METHODS We gave preoperative radiotherapy (total dose, 45 Gy) to 23 patients with locally advanced (T3/T4) rectal cancer. Concurrently, S-1 was given orally at a fixed dose of 80 mg/m2/day on Days 1-5, 8-12, 22-26, and 29-33, and irinotecan was given as a 90-min continuous i.v. infusion on Days 1, 8, 22, and 29. The dose of irinotecan was initially 40 mg/m2/day and gradually increased to determine the MTD and RD of this regimen. RESULTS Among the 4 patients who received 90 mg/m2 irinotecan, 2 had Grade 4 neutropenia and 1 had Grade 3 diarrhea. Because dose-limiting toxicity (DLT) occurred in 3 of the 4 patients, 90 mg/m2 irinotecan was designated as the MTD. Consequently, 80 mg/m2 irinotecan was given to 7 additional patients, with no DLT, and this was considered the RD. Of the patients who received irinotecan at the RD or lower doses, 6 (31.6%) had a complete pathologic response (Grade 3) and 9 (47.4%) underwent sphincter-preserving surgery. CONCLUSIONS With our new regimen, the MTD of irinotecan was 90 mg/m2, and the RD of irinotecan for Phase II studies was 80 mg/m2. Although our results are preliminary, this new neoadjuvant chemoradiotherapy was considered safe and active, meriting further investigation in Phase II studies.