Scientific advances can improve the lives of adults with intellectual disability, yet concerns that research participation may impose harm impede scientific progress. What counts as harmful can be subjective and perceptions of harm may vary among stakeholders. We studied perspectives on the harmfulness of research events among adults with intellectual disability, family members and friends, disability service providers, researchers, and Institutional Review Board members. We found considerable variance. For example, adults with intellectual disability see exclusion from research as more harmful, but most psychosocial harms as less significant than others. All stakeholders agree that having someone else make the participation decision is harmful. Findings provide insights into the concept of harm and ethical research with adults with intellectual disability. [This article was written with The Project ETHICS Expert Panel, which includes: Anna Carroll, Marty Cuddy, Micah Fialka-Feldman, Dan Flanigan, Pat Fratangelo, Lance Gonzalez, Michael Kennedy, Kathleen King, Chris Mansfield, Deb McGowan, Rachel Romer, Margaret Turk, Shquria Velez, Pamela Walker, and Priscilla Worral.]