Smoking causes multiple organ dysfunction. The effect of smoking on skeletal muscle protein metabolism is unknown. We hypothesized that the skeletal muscle protein synthesis rate is depressed in smokers compared with non-smokers. We studied eight smokers (20 cigarettes/day for 20 years) and eight non-smokers matched for sex (4 men and 4 women per group), age (65±3 and 63±3y, respectively; means±SEM) and body mass index (25.9±0.9 and 25.1±1.2 kg/m², respectively). Each subject underwent an intravenous infusion of stable isotope-labeled leucine in conjunction with blood and muscle tissue sampling to measure the mixed muscle protein fractional synthesis rate (FSR) and whole-body leucine rate of appearance (Ra) in plasma (an index of whole-body proteolysis), the expression of genes involved in the regulation of muscle mass (myostatin, a muscle growth inhibitor; MAFBx and MuRF-1, which encode E3 ubiquitin ligases in the proteasome proteolytic pathway) and that for the inflammatory cytokine tumor necrosis factor-alpha (TNF) in muscle, and the concentration of inflammatory markers in plasma which are associated with muscle wasting in other conditions. There were no differences between non-smokers and smokers in leucine Ra and plasma concentrations of inflammatory markers, or TNF mRNA in muscle, but muscle protein FSR was much less (0.037±0.005 vs. 0.059±0.005 %/h, respectively; P=0.004) and myostatin and MAFBx (but not MuRF-1) expression was much greater (by ~33% and 45%, respectivley; P<0.05) in the muscle of smokers than of non-smokers. We conclude that smoking impairs the muscle protein synthesis process and increases the expression of genes associated with impaired muscle maintenance.