To determine clinical effectiveness of strength training in children with spastic cerebral palsy. Prospective before and after trial in which subjects participated in a 6-week strength training program. All received before and after isometric strength evaluation of eight muscle groups in both lower extemities with a hand-held dynamometer, 3-D gait analysis at free and fast speeds, administration of the Gross Motor Function Measure (GMFM), and assessment of energy expenditure during gait. Pediatric rehabilitation center at a tertiary care hospital. Eleven children met inclusion criteria for participation. Six had spastic diplegia, were limited community ambulators, and demonstrated less than 50 percent of normal muscle strength. Five had spastic hemiplegia and demonstrated a 20 percent strength asymmetry in at least two muscles across extremities. Each group had significant strength gains in the muscles targeted. The entire cohort had higher gait velocity primarily as a result of increased cadence, with greater capacity to walk faster. GMFM Dimension 5 also improved, with no change in energy expenditure. Asymmetry in strength improved in hemiplegia, with no change in asymmetry in support times or joint motion across extemities. This study reinforced the relationship of strength to motor function in cerebral palsy and further demonstrated the effectiveness of strengthening in this population