A significant concern of parents and professionals is that adults with intellectual and developmental disabilities will go without regular educational–vocational activities. The authors examined predictors of such inactivity in individuals with intellectual and developmental disabilities, as well as how inactivity related to their sibling's well-being and the sibling relationship. Participants included 796 siblings of adults with intellectual and developmental disabilities who responded to a web-based survey. Nearly 13% of adults with intellectual and developmental disabilities were without daytime activities; these adults had more emotional–behavioral and health problems, were more underserved by the formal service system, and had parents who were less able to provide care. Although siblings of adults without activities reported more depressive symptoms, worse health, and less close sibling relationships, inactivity no longer predicted these problems after controlling for characteristics that predisposed adults with intellectual and developmental disabilities to have no activities.