Aims and Background Adjuvant 5-FU chemotherapy plus radiotherapy represents the standard treatment for radically resected rectal cancer at high risk of relapse according to the NIH Consensus Conference. The therapeutic gain was obtained with a high rate of severe treatment-related toxicity and a sub-optimal patient compliance with this regimen. Raltitrexed is a specific thymidylate synthase inhibitor with a convenient administration schedule, acceptable toxicity and radiosensitizing properties, as the published phase I trials in combination with radiotherapy have shown. The aim of this prospective multicenter phase II study was to evaluate the feasibility, gastrointestinal and hematological acute toxicity of raltitrexed in combination with radiotherapy in rectal cancer patients. Methods From September 2000 to June 2004, 50 patients with radically resected stage II-III rectal adenocarcinoma were treated. All patients were evaluable for compliance and acute toxicity. Within 45-60 days of surgery, each patient underwent concomitant adjuvant radiochemotherapy. Radiotherapy was administered to the pelvis (plus perineum after abdominoperineal resection) with photon beam energy exceeding 5 MV, 3-4 fields, 45 Gy/25 fractions/5 weeks plus a boost delivered to the site of resected disease with 3-4 fields, 9 Gy/5 fractions/1 week to a total dose of 54 Gy. The boost dose was administered after complete exclusion of the small bowel from the treatment volumes; if this was not possible a total dose of 50.4 Gy was given. Raltitrexed was administered intravenously at a dose of 3 mg/m² as a bolus injection on days 1 and 22 of radiotherapy one hour before treatment, for a total of two cycles. Each patient underwent weekly clinical evaluation and laboratory tests. Toxicity was assessed by the WHO scale. Results Forty-five (90%) patients completed the established treatment. Acute severe toxicity included grade III proctitis in 4/50 (8%), grade III-IV diarrhea in 4/50 (8%), grade III perineal dermatitis in 4/50 (8%) and grade III leukopenia in 2/50 (4%) patients; five patients (10%) experienced a transient grade III increase in their liver biochemistry values. Conclusions Our data related to acute toxicity and patient compliance proved the feasibility of this adjuvant radiochemotherapy treatment. A longer follow-up is necessary to evaluate the effectiveness of this new regimen in terms of disease-free and overall survival.