The aim of this study was to investigate the relation between muscle thickness of the quadriceps femoris muscle, knee joint function (spasticity and range of motion), and activity and participation measures in children and adolescents with cerebral palsy (CP). Thirty-eight children and adolescents with mild to severe CP (20 males, 18 females; mean age 12y 8mo [SD 3y 7mo], range 6-18y) participated. The severity and type of CP of participants covered all five levels of the Gross Motor Function Classification System and three types: spastic (quadriplegia, hemiplegia, and diplegia), athetotic, and hypotonic. The thickness of the quadriceps femoris muscle (MTQ) was measured from B-mode ultrasound images. Activity limitations were evaluated by the Gross Motor Function Measurement-66 (GMFM-66) and the Pediatric Evaluation of Disability Inventory (PEDI). Spasticity was assessed with the modified Ashworth scale (MAS). After adjustment for age and body mass index, the MTQ showed significant positive correlations with GMFM-66 and PEDI scores; however, there was no significant correlation with MAS ratings. The degree of knee flexion contracture correlated positively with the MAS rating of the knee flexor muscles and negatively with the MTQ. These results established the clinical relevance of assessment of muscle thickness across a broad spectrum of individuals with CP.