The potential value of microRNAs (miRNAs) as prognostic biomarkers are of interest. It is known that microRNA-21 (miR-21) is implicated in the promotion, proliferation and progression of several types of human cancers. However, the prognostic significance of miR-21 in each tumor stage of colorectal cancer (CRC) remains elusive. The objective of this study was to clarify the prognostic value of miR-21 for CRC patients at each tumor stage. The expression levels of miR-21 in the tumor tissues and normal adjacent tumor tissues of 306 CRC patients were determined by TaqMan microRNA assays. In order to clarify the miRNA profile in CRC tissues, miRNA arrays were examined. In this analysis, miR-21, miR-224, miR-96, miR-31 and miR-155 showed marked upregulation, and miR-21 showed the highest level. Upon comparison of clinicopathological factors, miR-21 expression showed significant association with depth of invasion, lymphatic and venous invasion, liver metastasis and Dukes' stage. In the Kaplan-Meier survival curve analysis of all patients, overall survival (OS) and disease-free survival (DFS) rates of the patients with high miR-21 expression were significantly worse than these rates in patients with low miR-21 expression. In the Kaplan-Meier analysis of each tumor stage, the DFS of patients with high miR-21 expression was significantly worse than patients with low miR-21 levels in Dukes' stage A tumors. In Dukes' stage B and C, patients with high miR-21 expression showed a significantly worse OS and DFS than patients with low miR-21 expression. In Dukes' stage D, patients with high miR-21 expression showed a significantly worse OS than patients with low miR-21 expression. In the Cox multivariate analysis, it was shown that miR-21 expressions in CRC tissues is an independent prognostic factor in Dukes' stage B, C and D. In conclusion, miR-21 expression may be a valuable biomarker for prediction of poor prognosis in CRC patients with Dukes' stage B, C and D.