Here we show that rhIL-7-hyFc dramatically enhances CAR T cell expansion, persistence, and anti-tumor efficacy, resulting in significantly prolonged survival of mice in CD19+ lymphoma and CD33+ leukemia xenograft models treated with human CAR T cells, and in immunocompetent mice bearing CD19+ tumors treated with murine CAR T cells. The gene discussed is CD33; the disease is neoplasm.