The purpose of this study was to determine the pretreatment serum squamous cell carcinoma antigen (SCC-ag) level as a generally applicable measurement in predicting and estimating the treatment outcome of patients with locally advanced SCC of the cervix. Three hundred fifty-two patients with stage IIB–IVA SCC of the cervix were managed with both external irradiation and high–dose rate intracavitary brachytherapy. A significantly higher median SCC-ag was seen in association with increasing stage, tumor size, and lymph node involvement. The difference in disease-free survival (DFS) between stages IIB and III patients was not statistically significant with SCC-ag level <2 ng/mL. In multivariate analysis, median SCC-ag level (≥6.0 ng/mL) and lymph node metastases had significant independent effects on absolute survival and DFS. A direct linear relationship (y = −2.932x + 84.896) existed between the median SCC-ag of groups distributed by pretreatment prognostic factors and the 5-year DFS rate. The 5-year DFS rate as a function of SCC-ag level defined by cervix size, lymph node status, and hydronephrosis was obtained from a formula combining risk scores and the baseline survival function. From the obtained formulas, we can objectively estimate the treatment outcome in patients with locally advanced squamous cell cervical cancer.