A 3-year-old with metastatic hepatoblastoma experienced complete cancer regression after receiving experimental GPC3-CAR T cell therapy, an immunotherapy that genetically engineers a patient's immune cells to target liver cancer. The boy, whose cancer had spread to his lungs and bones and recurred despite chemotherapy and surgery, remained disease-free 12 months after two infusions of the treatment. This case marks a significant advance in using CAR T cell therapy against solid tumors, though further trials are needed to confirm efficacy and safety.