['MicroRNAs, a class of noncoding RNAs, play roles in human diseases. MicroRNA-223 (miR-223) is reported to play critical roles in osteoclastogenesis. The purpose of this study was to analyze the expression pattern of miR-223 in rheumatoid arthritis (RA) synovium and examine the suppression of osteoclastogenesis from human peripheral blood mononuclear cells (PBMC) by overexpression of miR-223.', 'Expression of miR-223 in synovium from RA patients was analyzed by quantitative reverse transcription polymerase chain reaction (RT-PCR) and section in situ hybridization. MiR-223 was overexpressed in an osteoclastogenesis coculture system with PBMC and RA synovial fibroblast. At 3 weeks after transfection of double-stranded miR-223, the formation of tartrate-resistant acid phosphatase (TRAP)-stained multinucleated cells was analyzed to evaluate the inhibitory effect of miR-223 on osteoclastogenesis.', 'MiR-223 was more highly expressed in RA synovium than in osteoarthritis (OA) synovium due to the increased number of miR-223-positive cells in RA synovium. MiR-223 was expressed in the superficial and sublining layers, and macrophages, monocytes, and CD4 T cells also expressed miR-223. The number of TRAP-positive multinucleated cells was significantly decreased by overexpression of miR-223 in a dose-dependent manner. The expression of osteoclastogenesis marker genes was significantly down-regulated by miR-223 overexpression.'] Does overexpression of microRNA-223 in rheumatoid arthritis synovium control osteoclast differentiation?