In healthy individuals, postexercise protein supplementation increases muscle protein anabolism. In patients with muscular dystrophies, aerobic exercise improves muscle function, but the effect of exercise on muscle protein balance is unknown. Therefore, we investigated 1) muscle protein balance before, during, and after exercise and 2) the effect of postexercise protein-carbohydrate supplementation on muscle protein balance in patients with muscular dystrophies. In 17 patients [7 women and 10 men, aged 33 ± 11 yr (18–52), body mass index: 22 ± 3 kg/m² (16–26)] and 8 healthy matched controls [3 women and 5 men, age 33 ± 13 years (19–54), body mass index: 23 ± 3 kg/m² (19–27)], muscle protein synthesis, breakdown, and fractional synthesis rates (FSR) were measured across the leg using tracer dilution methodology on two occasions, with and without oral postexercise protein-carbohydrate supplementation. In patients, muscle protein breakdown increased in the recovery period (11 ± 1 μmol phenylalanine/min) vs. rest (8 ± 1 μmol phenylalanine/min, P = 0.02), enhancing net muscle protein loss. In contrast, postexercise protein-carbohydrate supplementation reduced protein breakdown, abolished net muscle protein loss, and increased the muscle FSR in patients (0.04 to 0.06%/h; P = 0.03). In conclusion, postexercise protein-carbohydrate supplementation reduces skeletal mixed-muscle protein breakdown, enhances FSR, resulting in a reduced net muscle loss in patients with muscular dystrophies. The findings suggest that postexercise protein-carbohydrate supplementation could be an important add-on to exercise training therapy in muscular dystrophies, and long-term studies of postexercise protein-carbohydrate supplementation are warranted in these conditions.