Breast conserving surgery is increasingly performed in early stage breast cancer patients. A certain propotion of these patients arc at substantial risk for systemic metastasis. However, there is no valid consensus about optimal sequencing of chemotherapy and radiation therapy. We conducted a randomized prospective study to investigate whether concurrent chemotherapy and radiation therapy after breast conservation surgery are associated with increased toxicity. Fifty-seven patients with stage I or II breast cancer were randomly assigned to receive CMF chemotherapy either simultaneously (n=37) or before (n=20) radiation therapy. Moist desquamation was the most common adverse effect which occurred in 46% (16/37) treated with concurrent chemotherapy and radiation therapy while 8 patients (38%) treated with sequential regimen had the finding. Difference between two groups was not statistically significant. Incidence of severe neutropenia (WBC<1,800) or abnormal elevation of liver enzymes was also not influenced by sequencing of adjuvant therapies. Arm edema was observed in 2 patients of concurrent group and was observed in 2 patients treated with sequential regimen. Two patients treated with concurrent regimen did not complete 6 cycles of chemotherapy while one patient with sequential regimen did not complete, Incidence of toxicity during chemotherapy was not altered by timing of radiation therapy, In conclusion, chemotherapy and radiation therapy can be given concurrently after breast conservation surgery in stage I or II breast cancer patients without increase of serious toxicity.