Dexamethasone (Dex), alone or in association with estrogens, is often illegally administered per os at very low-dosage as a growth promoter in beef cattle, with effects that are opposite to the muscle wasting and atrophy induced by repeated administration at therapeutic dosages. In vitro and in vivo studies have investigated the catabolic effects of Dex at therapeutic doses on skeletal muscle, demonstrating an increase in the expression of GDF8 (myostatin) gene, a well-known negative regulator of skeletal muscle mass, in a dose-dependent way. This suggested a direct role of myostatin in dexamethasone-induced muscle wasting. In the present study, an oligonucleotide microarray platform was used to compare expression profiles of beef cattle muscle in animals treated with either Dex or Dex plus 17- estradiol (Estr) administered at sub-therapeutic dosage, against untreated controls. Data analysis demonstrates that the expression profiles were strongly affected by Dex treatment with hundreds of genes up-regulated with relevant fold-change, whereas seven genes were down-regulated including the myostatin gene. On the contrary, the number of differentially regulated genes was lower in response to the addition of Estr to the Dex treatment. Differentially regulated genes were analysed to describe the effects of these treatments on muscle physiology, highlighting the importance of specific pathways (e.g. Wnt or cytokine signalling) and cellular processes (e.g. cell shape and motility). Finally, the observed differences in the expression profile will allow the development of indirect bio-markers to detect illegal Dex treatments in beef cattle using quantitative RT-PCR.