PURPOSE: Breast conserving surgery is increasingly performed in early stage breast cancer patients. A certain propotion of these patients are at substantial risk for systemic metastasis. However, there is no valid consensus about optimal sequencing of chemotherapy and radiation therapy. METHODS: We conducted a randomized prospective study to investigate whether concurrent chemotherapy and radiation therapy after breast conserving surgery are associated with increased toxicity. Fifty-seven patients with stage I or II breast cancer were randomly assigned to receive CMF chemotherpy either simultaneously with (n=37) or before (n=20) radiation therapy. RESULTS: Moist desquamation was the most common adverse effect which occurred in 46% (16/37) of the patients treated with concurrent chemotherapy and radiation therapy and 40% (8/20) of those treated with the sequential regimen. This difference between two groups was not statistically significant. Also the incidence of severe neutropenia (WBC<1,800) or abnormal elevation of liver enzymes was not influenced by the sequencing of the adjuvant therapies. Arm edema was observed in 2 patients of the concurrent group and in 2 patients of the sequential regimen. Two patients treated with the concurrent regimen did not complete 6 cycles of chemotherapy while only one patient treated with sequential regimen did not. The incidence of toxicity during chemotherapy was not altered by the timing of radiation therapy. CONCLUSION: Chemotherapy and radiation therapy can be given concurrently after breast conserving surgery in stage I or II breast cancer patients without increase in serious toxicity.