Background: Protein catabolism is one of the major side effects of corticosteroid treatment. An increase in muscle protein breakdown has been suggested as the main mechanism. It is unclear whether muscle protein synthesis plays a role in corticosteroid-induced protein catabolism. Subjects: To investigate whether corticosteroid treatment affects muscle protein fractional synthetic rate (FSR) in growing piglets, we infused 12 piglets (11.2-15.2 kg body weight) with 1-13C-labelled valine (2 mg/kg/h for 4h), and measured incorporation of labelled valine into hindleg muscle by continuous flow gas chromatography/combustion/isotope ratio mass spectrometry. Interventions: 6 piglets were treated with the corticosteroid dexamethasone (5 mg/kg for 7d), 6 piglets served as controls. Results: Muscle protein FSR was 2.81 ± 0.33%/d (mean ± SD) in controls. Corticosteroid treatment decreased FSR by 35% to 1.83± 0.29%/d (p<0.001). Conclusions: We conclude that corticosteroid treatment decreases muscle protein FSR in growing piglets. We suggest that this reduction in FSR plays a role in corticosteroid-induced whole body protein catabolism.