© 2015 Wiley Periodicals, Inc. Introduction: In this study we aimed to characterize muscle composition of the medial gastrocnemius in children with spastic cerebral palsy (SCP) using quantitative ultrasound. Methods: Forty children with SCP, aged 4-14 years, participated in this study. Children were grouped according to the gross motor function classification system (GMFCS I-V) and compared with a cohort of age- and gender-matched, typically developing children (TD; n=12). Ultrasound scans were taken of the medial gastrocnemius. Images were then characterized using grayscale statistics to determine mean echo intensity (EI) and the size and number of spatially connected homogeneous regions (i.e., blobs). Results: Significant differences in skeletal muscle composition were found between children with SCP and their TD peers. Children classified as GMFCS III consistently exhibited the highest EI and blob area. Conclusions: This study demonstrates altered tissue composition in children with SCP visualized using ultrasound. Further work is required to determine the pathophysiology contributing to these alterations in SCP.