Renewed interest in antifols for the treatment of childhood cancers has resulted from identification of novel antifols with broad spectrums of anti-cancer activity and from re-evaluation of the original clinical antifol, aminopterin. In this pre-clinical study we evaluated the in vitro activity of both traditional antifols (methotrexate, aminopterin) and novel antifols (pemetrexed, talotrexin) in childhood acute leukemias and lymphomas.