Although weakness has been identified in cerebral palsy (CP) in isolated muscle groups, the magnitude of weakness in multiple muscles and the patterns of weakness across joints have not been documented. The maximum voluntary contraction of eight muscle groups in the lower extremities of 15 children with spastic diplegia, 15 with spastic hemiplegia, and 16 age-matched peers was determined using a hand-held dynamometer. Children with spastic diplegia were shown to be weaker than age-matched peers in all muscles tested, as were the children with hemiplegia on the involved side, with strength differences also noted on the uninvolved side. Weakness was more pronounced distally in the groups with CP, and the hip flexors and ankle plantarflexors in spastic CP tended to be relatively stronger than their antagonists as compared with the strength ratios of the comparison group. In conclusion, children with spastic CP demonstrate quantifiable lower-extremity weakness and muscle imbalance across joints.