Physical inactivity is an increasing problem in the general population in society. However, in people with disabilities, inactivity is even more frequently reported. Physical activity on prescription (PAP) is a well-established method to enhance physical activity. The prescribed physical activity can be activities such as walking, cycling, swimming, or gardening, and should be performed over a longer period. In children with cerebral palsy, PAP has shown to be feasible to increase participation in physical activity and to reduce sedentary behaviour, and the habilitation services in the Region of Skåne have decided to offer PAP to all children and youth with disabilities. It is of great importance to study these interventions when applied on a broader group of patients than previously studied. The aim is to study the effects of an individualised and health-enhancing intervention in physically inactive people with autism, intellectual and physical disabilities. Also, the cost-effectiveness of PAP will be studied. 60 physically inactive children, aged 8- 17 years, and 20 adults, with autism, intellectual or physical disability will be included. The participants are recruited by their clinical physiotherapists, who also will be carrying out the PAP-intervention. The self-selected physical activity/activities may either be a physical activity organized by a club and/or an everyday activity such as walking a dog or riding a bicycle to school. Each participant fills in an activity logbook. Motivational interviewing will be used to support the participants. The Canadian Occupational Performance Measure can be used to identify what activities the participants are motivated to do, and to detect changes in the participants' perception of their performance of the activity. Quality of life will be monitored. Physical activity will be measured through the International Physical Activity Questionnaire (IPAQ) and a movement monitor (accelerometer). Study specific questionnaires will be filled in regarding costs and background information. Data on health care use of the participants will retrospectively be collected and studied using the health care database in Region Skåne. Data will be collected at 3 months prior to the intervention, just before the intervention starts, when the intervention is finished, and at 6, 12 and 24 months after the intervention. The study has been approved by the Swedish Ethical Review Authority.