Conclusions. In addition to the correlation of transforming growth factor (TGF)-β1 levels to the radiation toxicity, both Epstein–Barr virus (EBV) infection and postoperative status play roles in the change in plasma TGF-β1 levels in patients with head and neck cancers. Objectives. To assess the parameters involved in the change in plasma TGF-β1 level during concurrent chemoradiotherapy (CCRT). Patients and methods. Blood samples (n=307) before, during, and after treatment were obtained from 39 patients with head and neck cancers treated with definitive or adjuvant CCRT. In situ hybridization (ISH) was used to identify EBV-encoded RNA (EBER) in tissues from the primary tumor or metastatic lymph nodes. Plasma TGF-β1 level, white blood cell (WBC), and platelet count were assayed immediately before the first fraction of radiotherapy (RT), once a week during RT, and at the end of the RT course. The grades of mucositis and dermatitis were recorded weekly during CCRT. Results. Pretreatment TGF-β1 level and radiation toxicity were found to be significantly correlated with the increase of the plasma TGF-β1 level during CCRT (p<0.05), but correlation of platelet count was only found in patients whose tumors were EBV-positive (p=0.0042), and WBC count in those treated with postoperative adjuvant CCRT (p=0.004).