The aims of the current study were to assess the prevalence of SLC19A1 80 G>A and MTHFR 677 C>T genes polymorphisms as well as TS gene tandem repeats in the group of children treated due to ALL and its influence on Mtx pharmacokinetics and incidence of acute toxicities caused by HD-Mtx. This evidence concerns the gene TYMS and acute lymphoblastic leukemia.