Because adolescents are codified as incompetents, their health care decision-making authority often is encroached upon. When a teenager develops a serious disease such as cancer, others are even more likely to appropriate decision-making authority than under normal circumstances. Adolescents can contribute to decisions concerning their treatment provided certain guidelines are used. These guidelines stem from bioethical concerns and our understanding of the development in cognitive ability related to decision-making. This article summarizes considerations relating to each of these two areas. Because the clinical stage of the malignancy also affects bioethical concerns, the adolescent's role in decision-making at the time of initial diagnosis and recurrence are discussed.