The purpose of this study was to describe the toxicity of concurrent standard dose adjuvant radiation therapy (RT) and paclitaxel in a series of patients receiving primary breast cancer therapy. From June 1998 to April 1999, 20 patients with breast cancer received concurrent adjuvant radiation and paclitaxel. There were 16 patients (80%) with American Joint Committee on Cancer (AJCC) stage II disease and 4 with stage III disease. Eighteen patients, 12 postmastectomy and 6 breast conservation, were treated with definitive surgery followed by concurrent RT and paclitaxel. Two received concurrent neoadjuvant radiation and paclitaxel. All patients received a doxorubicin-containing combination prior to radiation and paclitaxel. RT was delivered concurrently with paclitaxel after the completion of all doxorubicin therapy, with all patients receiving at least two cycles of paclitaxel (175 mg/m2) every 3 weeks during RT. Toxicity was graded weekly according to Radiation Therapy Oncology Group criteria. Thirteen patients (65%) developed grade 2 or higher cutaneous toxicity. In the postmastectomy group, 6 of 12 patients (50%) developed grade 2 cutaneous toxicity, and 4 of 12 patients (33%) developed grade 3. RT was discontinued in 1 and placed on hold in 3 of these patients. In the breast-conservation group, 2 of 6 patients (33%) developed grade 3 toxicity. In the neoadjuvant group, 1 of 2 patients (50%) developed grade 3 toxicity. Four patients (20%) developed radiation pneumonitis, 2 of 12 (17%) in the postmastectomy group and 2 of 6 (33%) in the breast conservation group, with 2 requiring hospitalization and 1 a diagnostic open-lung biopsy. In this group of patients, standard dose concurrent radiation and paclitaxel resulted in a high incidence of cutaneous and pulmonary toxicity. Concurrent radiation and paclitaxel with these doses and schedule should be approached cautiously until further studies documenting its safety are completed.