In this issue of JAMA Pediatrics, Whittington et al1 present a comprehensive analysis of the projected long-term outcomes and cost-effectiveness of a chimeric antigen receptor T-cell therapy, tisagenlecleucel, for relapsed or refractory leukemia in children; this is the first gene therapy approved by the US Food and Drug Administration. The study uses a modeling approach and highlights 2 critical issues in defining value for an innovative treatment with limited evidence: the methodological challenges of applying economic evaluation techniques to rare pediatric conditions and reliably assessing value and affordability for very costly new treatments (in this case, $475 000 per patient).