We report on a 16-year-old girl with spastic quadriplegic cerebral palsy associated with premature birth and typical periventricular leukomalacia, who had a dramatic improvement in motor function after treatment with carbidopa/levodopa. Kinematic and electromyographic analyses of reaching movements demonstrate that levodopa decreased muscle co-contraction, decreased unwanted movements, and improved her ability to maintain a steady arm posture. These findings suggest that levodopa be considered as an adjunct therapy for the treatment of spastic quadriplegic cerebral palsy.