Smokers are more likely to have sarcopenia than non-smokers. However, the mechanism by which smoking decreases skeletal muscle maintenance, including possible effects on muscle protein synthesis and breakdown is unknown. We therefore tested the hypothesis that the rate of muscle protein synthesis is lower in smokers than in non-smokers. We measured the fractional rate of mixed muscle protein synthesis and whole-body leucine rate of appearance in plasma (Ra, an index of whole-body proteolysis) by using stable isotope-labeled tracer techniques in eight habitual smokers (≥20 cigarettes per day for ≥20 years) and eight non-smokers matched for sex (4 men and 4 women per group), age (65 ± 3 y and 63 ± 3 y; means ± SEM) and BMI (25.9 ± 0.9 and 25.1 ± 1.2 kg/m2). Furthermore we measured the expression of genes in muscle known to be related to the regulation of muscle mass, namely those for the proteolytic associated enzymes MaFBx and MuRF-1 as well as that for the cytokines TNFα and myostatin. We also measured the concentration of a variety of inflammatory markers in plasma (C-reactive protein, tumor necrosis factor-alpha, interleukin 6) since they have been shown to be negatively correlated with muscle protein synthesis rates in other conditions. There were no differences between non-smokers and smokers in plasma leucine concentration (109 ± 7 µM vs. 104 ± 6 µM; P = 0.59), leucine Ra (1.31 ± 0.09 µmol/kg BWmin vs. 1.23 ± 0.03 µmol/kg BW.min; P = 0.42) or in plasma concentrations of inflammatory markers. However, mixed muscle protein synthesis rate was markedly less in smokers compared with non-smokers (0.037 ± 0.005 vs. 0.059 ± 0.005 %/h, respectively; P = 0.004). Furthermore the expression of MaFBx and mysostatin mRNA in the muscle of the smokers was markedly elevated compared to that in the non-smokers (4.5 ± 1.6 & 11 ± 6.6-fold respectively). We conclude that habitual smoking inhibits muscle protein synthesis and increases the expression of genes associated with increased muscle proteolysis and inhibition of muscle maintenance. These may be mechanisms by which smoking increases the risk of age-associated loss of muscle mass.