In this article we explore decision-making about treatment when a child faces a life-threatening illness but conventional treatment presents substantial risk and uncertain benefit. When is it acceptable for parents to decide to use complementary and alternative medicine as an alternative, rather than a complement, to conventional care? We use the example of a young child suffering from progressive glycogen storage disease, for whom liver transplant offers the only prospect of a cure. Without a liver transplant, the disease usually results in death within a few years. However, experience using transplant to treat this illness has been limited, success is far from ensured, and the risks (including death and continued progression of the disease) are substantial. The child's parents, who are first-generation immigrants, consider the risks of the transplant unjustified because it still does not offer good prospects for a healthy future. They believe that traditional Chinese medicine could help remediate their daughter's disease. In the article we (1) review parents' obligation to make treatment decisions in the best interests of their child, (2) explain limits on parents' decision-making authority, (3) explore how "best interests" are determined, focusing on cases of serious illness for which conventional treatment is risky and benefit is possible but uncertain, (4) explain the standard of care that physicians must meet in advising about treatment, and (5) outline factors that clinicians and parents should take into account when making decisions.