Background: Earlier research has distinguished five domains of Quality of life (QoL) for people with ID: material well-being, development and activity, physical well-being, social well-being, and emotional well-being. We investigated parents' perspectives on these domains and QoL for children and young adults with ID and hypothesized that parents' well-being would be a predictive factor in QoL of their children with ID. Methods: Our longitudinal study administered questionnaires to parents at T[subscript 0] (n = 147) and T1 (n = 108). The inclusion criteria were: (i) the child's age 0-24 years and (ii) the child's intellectual disability (IQ less than 70 or IQ less than 85 in combination with behavioural problems). Results: Social well-being of parents (P less than or equal to 0.001), changes in parents' social well-being (P less than or equal to 0.01) and changes in children's social well-being (P less than or equal to 0.05) were strong predictors of QoL for children with ID. Emotional well-being of children with ID (P less than or equal to 0.01), changes in children's emotional well-being (P less than or equal to 0.01) and changes in emotional well-being of parents (P less than or equal to 0.05) also predicted QoL of children with ID. Material well-being of parents, and health, development and activity of the children were not predictors. Conclusion: Our study revealed that predictors of QoL in children and young adults with ID occurred in the following domains: physical well-being (children), social well-being (parents and children) and emotional well-being (parents and children).