Growing evidence suggests microRNAs (miRNAs) have an important role in tumorigenesis. MicroRNA-21 (miR-21) is up-regulated in many malignant tumors, including breast cancer. Its association with clinicopathologic features and expression of PTEN (phosphatase and tensin homolog deleted on chromosome 10), one of its target genes, in breast cancer has not been reported systematically. To further determine the potential involvement of miR-21 in breast cancer, we have evaluated the expression level of miR-21 by stem-loop real-time RT-PCR based on SYBR-Green I in human invasive ductal carcinoma of the breast, and we have correlated the results with clinicopathologic features and PTEN protein expression. Matched non-tumor and tumor tissues of 40 human invasive ductal carcinoma of the breast were analyzed for miR-21 expression by stem-loop real-time RT-PCR based on SYBR-Green I. Immunohistochemistry (IHC) was used to estimate PTEN expression in tumor tissue. The expression levels of miR-21 were correlated with PTEN and commonly used clinicopathologic features of breast cancer. The stem-loop real-time RT-PCR based on SYBR-Green I was sensitive and specific enough to detect miR-21. Expression levels of miR-21 were significantly higher in tumor tissues than the levels in matched non-tumor tissues (P=0.000). Expression of miR-21 was negatively correlated with expression of PTEN (P=0.013). Up-regulated miR-21 expression was associated with lymph node positivity (P=0.01), higher proliferation index (ki67>10%) (P=0.03) and advanced breast cancer TNM clinical stage (P=0.021). These findings suggest that PTEN is possibly one of the targets of miR-21 in breast cancer and high expression of mir-21 indicates a more aggressive phenotype.