Background: Chemotherapeutic resistance indicated the poor prognosis of colorectal cancer. Objective: Our study aimed to investigate the role of STAT3/cyclinD1 pathway in the chemotherapeutic resistance of colorectal cancer. Methods: We firstly measured the expression of cyclinD1 in the colorectal cancer tissues using immunohistochemistry in tissue microarray. Then cell viability and apoptosis were investigated in the HT-29 cell lines dealing with recombinant lentivirus and shRNA to increase or decrease cyclinD1 expression. Furthermore, luciferase and ChIP assays were applied to investigate whether STAT3 regulated cyclinD1 expression by binding to its promoter. Finally, we determined whether inhibition of STAT3 could decrease cyclinD1 and increase the chemotherapy sensitivity. Results: CyclinD1 expression was significantly increased in the cancer cells and high level of cyclinD1 indicated the poor prognosis. Inhibition of cyclinD1 decreased the cell viability assessed by MU and increased rate of apoptosis when exposed to 5-FU treatment while overexpression of cyclinD1 showed the reverse effect. ChIP assay showed that STAT3 directly bind to cyclinD1 promoter. Subclone of full promoter of cyclinD1 into pGL4 increased the luciferase activity while delete or mutation of any of STAT3 binding sites resulted in reductions of luciferase activity. Inhibition of STAT3 decreased cyclinD1 expression to decrease the cell viability and increase rate of apoptosis when exposed to 5-FU treatment. Conclusions: Inhibition of STAT3/cyclinD1 pathway increased the sensitivity of colorectal cancer cell to chemotherapy. (C) 2015 Elsevier Inc. All rights reserved.