The pharmacokinetics and pharmacodynamics of therapeutic drugs can greatly vary among individuals. For example, it is sometimes necessary to alter the treatment of childhood acute lymphoblastic leukemia from the standard protocol. Genetic variation is one important factor, which can exert a wide range of effects on sensitivities and responses to therapeutic agents. Thiopurine S-methyl transferase (TPMT) is a useful test for predicting 6-mercaptopurine (6-MP) sensitivity in Caucasians. However, it is not effective for predicting the 6-MP therapeutic responses of Japanese patients because the frequency of TPMT deficiency is lower in the Japanese population (approximately 1% versus approximately 10% in Caucasians). Recently, NUDT15 polymorphisms have been reported to be predictive factors contributing to responsiveness to thiopurine therapy in Asians. The associations between genetic variants and therapeutic responses have been reported in Western countries. However, questions remain about whether results studying other races are applicable to Japanese due to differences in genetic variant frequencies among races. To provide personalized therapy based on genetic factors, we need to ascertain the relationships between genetic variants and therapeutic responses in Japanese childhood acute lymphoblastic leukemia cases.