The objective of this study is to assess the effect of an exercise program of 1 year on cardiovascular health, obesity and diabetes, osteoporosis, physical fitness, mental health and quality of life in childhood cancer survivors. The investigators will recruit childhood cancer survivors aged 16 years and above from three Swiss paediatric oncology clinics and randomize them into an intervention and a control group. The intervention group will be asked to increase physical activity for 1 year by at least 2.5 hours of intense physical activity weekly. Regular feedback will be given via a step counter, an online activity diary, and by the centre staff. The control group participants will keep their activity level constant. All participants will be seen after 3, 6 and 12 months to assess health and quality of life parameters over one year. After 1 year, the control group is offered to receive the same intervention to profit as well from an active lifestyle.