In the present study, we explored effects of the ABCC2 −24C>T polymorphism (rs717620) on MTX plasma concentrations in a relatively large childhood ALL patient population (n = 112) treated with high-dose MTX, and for the first time assessed the association between the ABCC2 −24C>T polymorphism and MTX toxicities. The gene discussed is ABCC2; the disease is acute lymphoblastic leukemia.