Concurrent chemoradiotherapy is reported to have a fair clinical outcome with organ preservation for patients with squamous cell carcinoma of the head and neck (SCCHN). The aim of this study was to determine whether biological markers are related to proliferative activity or apoptosis of tumor cells and whether clinical factors are associated with a clinical outcome in SCCHN patients treated with concurrent chemoradiotherapy. Immunostaining with antibodies specific for p53, bcl-2, bax, and MIB-1 was performed to evaluate expression of these proteins in formalin-fixed, paraffin-embedded specimens of 111 SCCHN patients treated with concurrent chemoradiotherapy (carboplatin, 100 mg/m2, four to six times every week; total radiation therapy dose of 40-65 Gy over 4-6.5 weeks). Multivariate analysis indicated that nodal status was a significant indicator of overall survival (OS; P = 0.001) and locoregional control (LRC; P = 0.002). In a univariate analysis, patients with a low MIB-1-positive index (< 40%) had better OS than those with a high MIB-1-positive index (> or = 40%; P = 0.013), although the difference was not statistically significant in a multivariate analysis (P = 0.060). Patients with bcl-2-positive tumors had better LRC than those with bcl-2-negative tumors, based on a multivariate analysis (P = 0.017). No statistically significant association was found between p53 or bax expression and clinical outcome. These results indicate that nodal status is the major prognostic factor in SCCHN patients treated with concurrent chemoradiotherapy. In addition, our findings suggest that bcl-2 positivity is associated with better LRC and that the proliferative activity of tumor cells might be prognostic for OS.