A prospective study of the families of young severely disabled children applying to a non-governmental organization for financial support examined the respective contributions of a child's intrinsic impairment and the family's environment to their participation. The study population comprised the families of 600 children of median age 2 years 8 months (range 1y-12y 1mo), of whom 46% had autistic or severe behaviour difficulties, 12% other learning disabilities, 6.8% cerebral palsy, and 10% non-neurological causes of severe disability. Intrinsic impairment was measured with the Health Utilities Index (HUI); environmental factors using a novel instrument, the European Child Environment Questionnaire (ECEQ); and participation of child and family using the Lifestyle Assessment Questionnaire. Principal components analysis was used to examine interrelationships. A five-component model explains 50% of the variance in the ECEQ. The first component, ECEQ1, reflects the support a family receives from professional services and family. HUI and ECEQ1 were independently associated with participation (p<0.0001) with comparable effect sizes. We conclude that the participation of young severely disabled children is affected to similar extents by intrinsic impairment and environmental factors such as services, support, and attitudes.