To determine efficacy of pediatric Constraint-Induced Movement therapy, 20 children with congenital hemiparesis (ages 2 to 6 years) were randomly assigned to receive the treatment or usual care. Controls crossed over to the therapy after 6 months. Children receiving the therapy first exhibited emergence of more new classes of motor patterns and skills (eg, crawling, thumb-forefinger prehension; 6.4 vs 0.02, P < .0001, effect size d = 1.3), and demonstrated significant gains in spontaneous use of the more affected arm at home (2.2 vs 0.1, P < .0001, d = 3.8) and in a laboratory motor function test. Depending on the measure, benefits were maintained (range, no loss to 68% retention over 6 months). When controls crossed over to the therapy, they exhibited improvements as great as or greater than those receiving therapy first. Thus, Constraint-Induced Movement therapy appears to be efficacious for young children with hemiparesis consequent to congenital stroke.