Glucocorticoids are the cornerstone of therapy in patients with idiopathic inflammatory myopathies (IIM), despite adverse effects and suboptimal therapy success rates. Glucocorticoids are used in patients with IIM to suppress inflammatory and immune responses implicated in the pathogenesis of these diseases. Nevertheless, potential inhibitory effects of glucocorticoids on skeletal muscle mass, myogenesis and immune responses that promote skeletal muscle regeneration after muscle injury warrant attention. Glucocorticoids lead to skeletal muscle catabolism by modulating major pathways involved in regulating muscle mass. Glucocorticoids also inhibit muscle regeneration by decreasing myogenic cell proliferation and differentiation. Finally, glucocorticoids might have inhibitory effects on immune cells that have been shown to be an important component of the muscle regenerative response. Better understanding of the signalling pathways involved in restorative versus adverse effects of glucocorticoids in IIM could yield additional insight into the aetiopathogenesis of persistent muscle weakness in patients with IIM after glucocorticoid treatment, and help in the development of novel, targeted treatment options with fewer adverse effects. © 2012 Macmillan Publishers Limited. All rights reserved.