Objective. Constraint-Induced Movement (CI) therapy has been found to be a promising treatment for substantially increasing the use of extremities affected by such neurologic injuries as stroke and traumatic brain injury in adults. The purpose of this study was to determine the applicability of this intervention to young children with cerebral palsy. Methods. A randomized, controlled clinical trial of pediatric CI therapy in which 18 children with diagnosed hemiparesis associated with cerebral palsy (7-96 months old) were randomly assigned to receive either pediatric CI therapy or conventional treatment. Pediatric CI therapy involved promoting increased use of the more-affected arm and hand by intensive training (using shaping) of the more-impaired upper extremity for 6 hours/ day for 21 consecutive days coupled with bivalved casting of the child's less-affected upper extremity for that period. Children's functional upper-extremity skills were assessed in the laboratory (blinded scoring) and at home (parent ratings) just prior, after, and 3 weeks post-treatment. Treated children were followed for 6 months. Results. Children receiving pediatric CI therapy compared with controls acquired significantly more new classes of motoric skills (9.3 vs 2.2); demonstrated significant gains in the mean amount (2.1 vs 0.1) and quality (1.7 vs 0.3) of more-affected arm use at home; and in a laboratory motor function test displayed substantial improvement including increases in unprompted use of the more-affected upper extremity (52.1% vs 2.1% of items). Benefits were maintained over 6 months, with supplemental evidence of quality-of-life changes for many children. Conclusion. Pediatric CI therapy produced major and sustained improvement in motoric function in the young children with hemiparesis in the study.