This research examines the applicability of the Theory of Planned Behavior in predicting supportive behaviors by parents and adult siblings of immediate relatives with intellectual disability. Participants were 67 parents and 63 siblings whose immediate relatives with intellectual disability resided in two institutional care facilities. Three aspects of supportive assistance behavior were evaluated: home visits, visits to the institution, and the relationship with the caregiving staff. Findings indicated that subjective norms held by siblings and parents predicted the frequency of home visits. Perceived behavioral control predicted the frequency of contact between siblings and staff. Thus, the applicability of the theory with respect to supportive behavior by the parents and siblings of immediate relatives with intellectual disability was partly substantiated by our findings. The findings are discussed with respect to the applicability of Theory of Planned Behavior. (Contains 1 figure and 6 tables.)