Abilities of adults with mild, moderate, or no mental retardation to understand hypothetical treatments was investigated using the Assessment of Consent Capacity–Treatment developed for this study based on Appelbaum and Roth's psycholegal consent standards. Performance in all groups decreased with increasing psycholegal complexity of consent decision-making. Most adults with mild and no mental retardation and almost half of adults with moderate mental retardation were able to make and justify treatment choices and fully or partially understand treatment information. Most adults without mental retardation, 50% with mild, and 18% with moderate mental retardation were able to partially appreciate relevance of treatment choice to patient's situation and weigh treatment risks against benefit. Implications of findings for patient rights are discussed.