New Findings r What is the central question of this study?It remains uncertain whether significant fat-free mass wasting occurs early after coronary artery bypass graft surgery, and the aetiology of this wasting in these particular conditions is unexplored. r What is the main finding and its importance?Significant fat-free mass wasting is present after coronary artery bypass graft surgery, and this wasting effect is greater in younger patients and in patients with greater increments in blood cortisol-to-testosterone ratios after surgery.The magnitude and aetiology of muscle wasting early after coronary artery bypass graft (CABG) surgery remains unknown. In the present study, we assessed changes in fat-free mass early after CABG surgery and explored the possible aetiology (relationships with postsurgical changes in blood hormones, insulin resistance, subject characteristics and inflammation) for these changes. Fat-free mass was assessed before and 23 (range: 25) days after CABG surgery in 25 subjects. Blood testosterone, cortisol, insulin-like growth factor-1, growth hormone, sex hormone-binding globulin, glucose, insulin, C-peptide and C-reactive protein concentrations were determined, and free androgen index, cortisol-to-testosterone ratio and HOMA-IR index were all calculated before surgery, during the first 3 days after surgery and at reassessment of body composition. Relationships between changes in fat-free mass and changes in blood parameters after surgery or subject characteristics were studied. After surgery, free androgen index and blood sex hormone-binding globulin, testosterone and insulin-like growth factor-1 concentrations decreased significantly, while HOMA-IR index, cortisol-to-testosterone ratio, blood growth hormone, insulin and C-reactive protein concentrations increased significantly (P < 0.0025, observed α > 0.80). Whole-body fat-free mass decreased significantly [by −1.9 (range: 9.1) kg, P < 0.0025, observed α = 0.99] after surgery. According to regression analysis, greater absolute loss of fat-free mass was observed after CABG surgery in subjects who were younger, who experienced a greater increase in blood cortisol-to-testosterone ratio after surgery and/or who underwent earlier reassessment of body composition (P < 0.05). Significant decrements in fat-free mass were observed early after CABG surgery, especially in younger subjects and/or 819 Muscle wasting after coronary artery bypass graft surgery subjects with elevated blood cortisol-to-testosterone ratios after surgery. Interventions to preserve fat-free mass soon after CABG surgery are thus warranted.