3605 Background: The anti-tumor activity of 5-fluorouracil (5-FU) has been related to its ability to induce apoptosis. Therefore, we assessed the predictive value of phenotypic expression of key apoptotic molecules in colorectal adenocarcinomas (CRC) of patients treated with 5-FU-based adjuvant therapies. METHODS: Archival tissues of CRCs from 56 patients who received a complete regimen of 5-FU-based chemotherapy after surgery, and 56 age, gender, ethnicity, tumor stage, tumor location, and tumor differentiation matched patients who had undergone only surgery (without any pre- or post-surgery chemo- or radiation therapy), were evaluated for immunophenotypic expression of Bax, Bcl-2 and nuclear accumulation of p53 (p53(nac)). Immunophenotypic expression of these markers was categorized into low and high expressors and cut-point values were determined based on their expression in benign epithelium. These tumors Ability of these markers in predicting the efficacy of 5-FU-based treatment was assessed by estimating overall survival utilizing the Kaplan-Meier and Cox proportional hazards methods. RESULTS: There was no significant difference in overall survival rates between the two patient categories (logrank P=0.487). However, a better survival was observed for patients who received chemotherapy when their CRCs lacked Bax expression; in contrast, patients with high Bax expression CRCs had worse survival when they received chemotherapy (logrank P=0.016). Surgically treated patients with CRCs lacked Bax expression had 5.33 times higher mortality than those with high Bax expression (HR, 5.33; CI: 1.78-15.94), when controlled for tumor stage and other confounding variables. Bcl-2 or p53(nac) had no predictive value in either patient group. CONCLUSIONS: These preliminary findings are the first to provide good evidence to suggest that patients with CRCs lacking Bax phenotypic expression benefit from 5-FU-based adjuvant therapies but not those with high Bax expression (Supported by NIH/NCI R01-CA98932-01). No significant financial relationships to disclose.