PURPOSE: The optimal sequence of chemotherapy (CT) and radiotherapy (RT) remains uncertain although both can reduce breast cancer recurrence after breast-conserving surgery (BCS). The current study was performed to evaluate whether concurrent RT with CT increases chemotherapy-associated toxicities. METHODS: Two hundred and thirty-eight patients with stage I and II breast cancers were prospectively allocated concurrent CT and RT (N=133) or sequential CT and RT (N= 105) after BCS. In the sequential group, the RT was started after completion of 3 cycles of CT with an additional 3 cycles of CT delivered after the RT. All patients underwent intravenous CMF chemotherapy composed of cyclophosphamide (500 mg/m2), methotrexate (50 mg/m2) and 5-FU (500 mg/ m2), every 3 weeks for 6 cycles following surgery. RESULTS: There were no significant differences between the two groups with regard to the grade 3 or 4 hematologic toxicities during chemotherapy or in abnormal liver enzyme elevation. Radiation related adverse effects, such as moist desquamation and pneumonitis symptom, were no different between the two groups. During the median 42 month follow-up period, range 16- to 60 months, 18 (13.5%) and 20 (19.1%) patients in the concurrent and sequential groups had systemic recurrences of breast cancer. The disease-free survival and local recurrence rates were no different between the two groups. CONCLUSION: Concurrent CT and RT were not associated with an increased toxicity and reasonable cosmetic results were achieved in this current study. The current study indicates that concurrent RT with CT after BCS is a feasible treatment modality, with the advantage of a shortening treatment time.