The Perceptions of Parental Illness Questionnaire was developed based on interviews with 15 adolescents with a parent with multiple sclerosis and refined using cognitive interviews. In total, 104 adolescents with a parent with multiple sclerosis then completed the Perceptions of Parental Illness Questionnaire and adjustment measures at two time points 6 months apart. Principal component analysis resulted in 11 Perceptions of Parental Illness Questionnaire sub-scales. Mixed-effect models showed that adolescents’ perceptions of parental multiple sclerosis at baseline rather than disease severity were associated with their psychosocial well-being 6 months later. The results indicate that Perceptions of Parental Illness Questionnaire may be a reliable and valid measure of adolescents’ representations of parents’ multiple sclerosis. Further studies are needed to replicate these findings with other illness groups.