Background. Muscle architecture is known to be predictive of muscle function. However, it is unknown whether this relationship is similar in children and adolescents with and without cerebral palsy (CP). Objective. The objective of this study was to determine whether the architecture of the rectus femoris (RF) and vastus lateralis (VL) muscles was predictive of maximum voluntary knee extensor torque in children and adolescents with and without CP and whether these measures were related to activity and participation levels. Design. A case-control design was used. Methods. Eighteen participants with CP (mean age=12.0 years, SD=3.2) at Gross Motor Function Classification System (GMFCS) levels I through IV and 12 agematched peers with typical development (mean age=12.3 years, SD=3.9) were evaluated. Muscle thickness, fascicle length, and fascicle angle of the RF and VL muscles were measured with 2-dimensional, B-mode ultrasound imaging. The activity and participation measures used for participants with CP were the Pediatric Outcomes Data Collection Instrument (PODCI) and the Activities Scale for Kids, Performance Version (ASKp). Results. When age and GMFCS level were controlled for, VL muscle thickness was the best predictor of knee extensor isometric torque in the group with CP (R 2=.85). This prediction was similar to the prediction from VL muscle thickness and age in participants with typical development (R2=.91). Rectus femoris muscle fascicle length was significantly correlated with the Sports and Physical Functioning Scale of the PODCI (ρ=.49), and VL muscle fascicle angle was correlated with the Transfers and Basic Mobility Scale of the PODCI (r=.47) and with ASKp Locomotion subdomain (r=.50). Limitations. A limitation of this study was the small sample size. Conclusions. Ultrasound measures of VL muscle thickness, adjusted for age and GMFCS level, were highly predictive of maximum torque and have the potential to serve as surrogate measures of voluntary strength (force-generating capacity) in children and adolescents with and without CP.