Expression of microRNA (miRNA) in the skin in dermatomyositis has not previously been studied in detail. In this study, we performed miRNA array analysis using miRNAs purified from dermatomyositis-involved skin and normal skin, and found that several miRNAs were up- or down-regulated in dermatomyositis skin. Among them, we focused on miR-7, one of the most down-regulated miRNAs in dermatomyositis skin. Total miRNAs were purified from serum, and hsa-miR-7 levels were measured with quantitative real-time PCR using the specific primer. Serum levels of miR-7 were significantly decreased in patients with dermatomyositis compared with normal subjects or patients with other autoimmune diseases. Thus, serum miR-7 levels might be a possible diagnostic marker for dermatomyositis. Clarifying the up- or down-stream events of down-regulated miR-7 in patients with dermatomyositis may lead to further understanding of the disease and a new therapeutic approach.