Earlier studies have shown that the progressive, unrelenting muscle loss associated with Duchenne Muscular Dystrophy (DMD) involves an imbalance between the rates of synthesis and degradation of muscle proteins. Although previous studies have suggested that oxandrolone may be beneficial in DMD, the mechanism of action of oxandrolone on muscle in DMD remains unclear. In order to address these issues, we combined stable isotope studies, and gene expression analysis to measure the fractional synthesis rate (FSR) of myosin heavy chain (MHC), the key muscle contractile protein, the transcript levels of the isoforms of MHC, and global gene expression profiles in four children with DMD before and after 3 months of treatment with oxandrolone. Gastrocnemius muscle biopsies and blood samples were collected during the course of a primed, 6 hour continuous infusion of L-[U-¹³C]leucine, on two separate occasions, before and after the 3-month treatment with oxandrolone (0.1mg/kg/day). Gene expression analysis was done with microarrays and RT/qPCR. In response to the treatment, MHC synthesis rate increased 42% and this rise was accounted for, at least in part, by an up-regulation of the transcript for MHC8 (perinatal MHC). Gene expression data suggested a decrease in muscle regeneration as a consequence of oxandrolone therapy, presumably due to a decrease in muscle degeneration. These findings suggest that 1) oxandrolone has a powerful protein anabolic effect on a key contractile protein and 2) larger and longer-term studies are warranted to determine if these changes translate into meaningful therapy for these patients.