OBJECTIVE: To determine the factors associated with tumor volume response to radiotherapy (RT) in cervical cancer patients, and the relationship between the tumor volume response and alteration of the expression of biological markers during RT. METHODS: Twenty consecutive patients with cervical squamous cell carcinoma who received definitive RT were enrolled. Tumor volumes were calculated by MRI examinations performed at the start of RT (pre-RT), at the fourth week of RT (mid-RT), and 1 month after RT completion (post-RT). Two serial punch biopsies were performed at pre- and mid-RT, and immunohistochemical staining was performed for cyclooxygenase (COX)-2 and epidermal growth factor receptor (EGFR). RESULTS: For the pre-RT evaluation, fourteen (70%) and eleven (55%) patients showed positive immunoreactivity for COX-2 and EGFR, respectively. Among the seven patients whose median percentage residual tumor at mid-RT (V2R) was greater than 0.5, seven (100%, p=0.0515) and five (71.4%, p=0.3742) patients showed positive immunoreactivity for COX-2 and EGFR, respectively. The logistic regression analysis showed that positive immunoreactivity for both COX-2 and EGFR at pre-RT were associated with V2R (p=0.0782). For the mid-RT evaluation, eight cases showed an interval increase in the distribution of immunoreactivity for COX-2, and six out of the eight patients had a V2R greater than 0.5 (p=0.2222). CONCLUSION: The poor mid-RT tumor response was associated with the coexpression of COX-2 and EGFR.