Aim: Optimal physical behaviour is important, as physical inactivity contributes to functional deterioration and reduced social participation. Nevertheless, research showed that persons with cerebral palsy (CP) have low physical activity levels. The objective of this study is to evaluate the effectiveness of a lifestyle intervention programme on physical behaviour. Method: Fifty-seven persons (36 completed the total study) with spastic CP (age range 16 to 25y; 27 males, 30 females), classified as Gross Motor Function Classification System levels I-IV were included in this randomized controlled trial. Twenty-nine participants had a unilateral CP and 27 had a bilateral CP. A 6-month lifestyle intervention consisting of fitness training and counselling on physical behaviour and sports participation was evaluated. Physical behaviour was objectively measured using ambulatory activity monitors. Self-reported physical activity was determined using the Physical Activity Scale for Individuals with Physical Disabilities. Results: The intervention did not affect the objectively measured physical activity during the intervention (beta=0.34, CI=-1.70 to 2.37) or at follow-up (beta=0.30, CI=-1.99 to 2.59). Self-reported physical activity was positively affected during the intervention period (beta=7.61, CI=0.17-15.05); however, this effect was not present at follow-up (beta=3.65, CI=-3.05 to 10.36). Interpretation: The lifestyle intervention was ineffective in eliciting a behavioural change towards more favourable physical behaviour in adolescents and young adults with spastic CP.