The physiologic response to trauma results in the efflux of large amounts of amino acids from skeletal muscle. This is extreme in large burn injuries. Protein kinetic studies, although useful in determining the rates of protein synthesis and breakdown, do not provide information about muscle loss. This study determined the contribution of muscle protein to whole-body protein breakdown in children throughout their course of burn injury. Children aged 0 to 18 years with initial burn size ≥30% TBSA underwent ¹⁵N glycine and 3 methylhistidine (3MH) analysis during three phases of care: A, early acute; B, wound closure; and C, convalescence. Muscle protein breakdown was estimated using a factor of 4.2 μmol 3MH per 1 g of mixed protein. Twenty-two patients with a mean of 54.5 ± 20.1% TBSA burn were studied. Protein balance did not change remarkably and remained positive by 2 g/kg during hospitalization. However, muscle protein breakdown dropped from 1.1 to 0.6 g/kg with wound closure (P < .0001), representing a decrease in the contribution of muscle protein to whole-body protein breakdown from 20 to 7%. Ten patients returned for a third measurement after discharge. Although protein turnover was high, muscle breakdown was consistent with 3MH values reported in healthy children. Serial determination of 3MH excretion is a simple way to track muscle catabolism throughout burn injury. Our data suggest that despite accelerated protein turnover, muscle catabolism significantly decreases with wound closure and begins to normalize around discharge. In convalescence, 3MH excretion is comparable with normal children.