AIM:To investigate the predictive value of Ki67 and p53 and their correlation with thymidylate synthase(TS) gene expression in a rectal cancer patient cohort treated according to a standardized recommended neoadjuvant treatment regimen.METHODS:Formalin fixed,paraffin embedded pretherapeutical tumor biopsies (n=22) and posttherapeutical resection specimens(n=40)from patients with rectal adenocarcinoma (clinical UICC stage Ⅱ/Ⅲ)receiving standardized neoadjuvant 5-fiuorouracil(5-FU)based chemoradiotherapy were studied for Ki67 and p53 expression by immunohistochemistry and correlated with TS mRNA expression by quantitative TaqMan realtime PCR after laser microdissection.The results were compared with histopathological tumor regression according to a standardized semiquantitative score grading system.RESULTS:Responders(patients with high tumor regression)showed a significantly lower Ki67 expression than non-responders in the pre-therapeutical tumor biopsies (81.2% vs16.7%;P＜0.05) as well as in the post-therapeutical resection specimens (75.8%vs14.3%;P＜0.01).High TS mRNA expression was significantly correlated with a high Ki67 index and low TS mRNA expression was significantly correlated with a low Ki67 index in the pre-therapeutical tumor biopsies (corr.coef.=0.46;P＜0.01)as well as in the posttherapeutical resection specimens (corr.coef.=0.40;P＜0.05).No significant association was found between p53 and TS mRNA expression or tumor regression.CONCLUSION:Ki67 has,like TS,predictive value in rectal cancer patients after neoadjuvant 5-FU based chemoradiotherapy.The close correlation between Ki67 and TS indicates that TS is involved in active cell cycle processes.