This study was a test for developmental differences in competency to make informed treatment decisions. 96 subjects, 24 (12 males and 12 females) at each of 4 age levels (9, 14, 18, and 21), were administered a measure developed to assess competency according to 4 legal standards. The measure included 4 hypothetical treatment dilemmas and a structured interview protocol. Overall, 14-year-olds did not differ from adults. 9-year-olds appeared less competent than adults with respect to their ability to reason about and understand the treatment information provided in the dilemmas. However, they did not differ from older subjects in their expression of reasonable preferences regarding treatment. It is concluded that the findings do not support the denial of the right of self-determination to adolescents in health-care situations on the basis of a presumption of incapacity. Further, children as young as 9 appear able to participate meaningfully in personal health-care decision making.