Objective To evaluate both the short- and long-term effectiveness of a lifestyle intervention on physical fitness in adolescents and young adults with cerebral palsy (CP). Design Single-blind, randomized controlled trial. Setting University hospitals and rehabilitation clinics. Participants Adolescents and young adults (N=57) with spastic CP classified in Gross Motor Function Classification System levels I through IV; of these, 42 completed the study. Intervention A 6-month lifestyle intervention consisting of physical fitness training combined with counseling sessions focused on physical behavior and sports participation. Main Outcome Measures Physical fitness, including measures of cardiopulmonary fitness, muscle strength, and body composition. Results Favorable short- and medium-term effects were found for peak oxygen consumption, oxygen consumption, and load on the anaerobic threshold and waist circumference. Favorable long-term effects were found for sum of skinfolds, systolic blood pressure, and total cholesterol. Conclusions This exploratory study showed that the lifestyle intervention was effective in improving cardiopulmonary fitness and body composition. Effects of body composition were maintained in the long term. However, the intervention needs to be optimized to increase muscle strength and for long-term retention of effects on aerobic capacity. © 2014 by the American Congress of Rehabilitation Medicine.