OBJECTIVE: To compare the effects of early and late treatment and to evaluate the contributing factors for the therapeutic outcome in cerebral palsy children. METHOD: Three hundred twenty four children with delayed development including cerebral palsy who were admitted to the Department of Rehabilitation at Yonsei University Medical Center from January 1992 to December 1995 were studied. They were divided into two groups according to the initiation of treatment: early and late treatment groups. Early treatment was defined as treatment started at or before 6 months and the late treatment was after 6 months. They were also divided into groups according to the diagnosis, responsiveness to treatment and initial motor quotient. And then the motor developments during the follow-up period between each groups were compared as the effects of treatment. RESULTS: The development of the motor milestone was faster in the early treatment group than in the late treatment group. The group which initially showed a higher motor quotient had a higher response rate to the treatment and a faster development of the motor milestone than the group with a lower motor quotient. CONCLUSION: These results suggest that the initiation of treatment and the severity of delayed development are important contributing factors for an outcome of treatment.