Question: In children with cerebral palsy, does a 6-month physical activity stimulation program improve physical activity, mobility capacity, fitness, fatigue and attitude towards sports more than usual paediatric physiotherapy? Design: Multicentre randomised controlled trial with concealed allocation, blinded assessments and intention-to-treat analysis. Participants: Forty-nine walking children (28 males) aged 7-13 years with spastic cerebral palsy and severity of the disability classified as Gross Motor Function Classification System level I-III. Intervention: The intervention group followed a 6-month physical activity stimulation program involving counselling through motivational interviewing, home-based physiotherapy, and 4 months of fitness training. The control group continued their usual paediatric physiotherapy. Outcome measures: Primary outcomes were walking activity (assessed objectively with an activity monitor) and parent-reported physical activity (Activity Questionnaire for Adults and Adolescents). Secondary outcomes were: mobility capacity, consisting of Gross Motor Function Measure-66 (GMFM-66), walking capacity and functional strength, fitness (aerobic and anaerobic capacity, muscle strength), self-reported fatigue, and attitude towards sport (child and parent). Assessments were performed at baseline, 4 months, 6 months and 12 months. Results: There were no significant intervention effects for physical activity or secondary outcomes at any assessment time. Positive trends were found for parent-reported time at moderate-to-vigorous intensity (between-group change ratio = 2.2, 95% CI 1.1 to 4.4) and GMFM-66 (mean between-group difference = 2.8 points, 95% CI 0.2 to 5.4) at 6 months, but not at 12 months. There was a trend for a small, but clinically irrelevant, improvement in the children's attitudes towards the disadvantages of sports at 6 months, and towards the advantages of sports at 12 months. Conclusions: This physical activity stimulation program, that combined fitness training, counselling and home-based therapy, was not effective in children with cerebral palsy. Further research should examine the potential of each component of the intervention for improving physical activity in this population. Trial registration: NTR2099. [Van Wely L, Balemans ACJ, Becher JG, Dallmeijer AJ (2014) Physical activity stimulation program for children with cerebral palsy did not improve physical activity: a randomised trial. Journal of Physiotherapy 60: 40-49]. © 2014 Australian Physiotherapy Association.