['To identify microRNAs (miRNAs) that are overexpressed in renal cell carcinoma (RCC) and characterize the functional role of miR-21.', 'The miRNA expression profiles between RCC tissue and adjacent normal tissue were compared using microarray analysis. The differential expression of miR-21 was validated by real-time polymerase chain reaction (RT-PCR). 786-O RCC cells were transfected with miR-21 mimic, miR-21 inhibitor, or negative controls and cell proliferation, apoptosis and cell cycle were examined by MTT assay and flow cytometry. The expression of programmed cell death 4 (PDCD4) and tropomyosin 1 (TPM1) was detected by RT-PCR and Western blot analysis.', 'Compared to adjacent normal tissue, 10 human miRNAs were significantly upregulated and 7 were downregulated in RCC tissue. RT-PCR confirmed that miR-21 was significantly overexpressed in RCC tissue. In vitro expression of miR-21 mimic promoted the growth of 786-O cells, whereas miR-21 inhibitor inhibited cell growth by inducing apoptosis and cell cycle arrest at S phase. Furthermore, miR-21 mimic or inhibitor significantly reduced or increased the expression of PDCD4 and TPM1.'] Is microRNA-21 overexpressed in renal cell carcinoma?