γ-Glutamyl hydrolase (GGH) plays a central role in folate metabolism and antifolate action. Polymorphism in the human GGH gene (GGH) was associated with efficacy and side effects of methotrexate for treatment of acute lymphoblastic leukemia (ALL). This study aimed to identify polymorphisms of GGH in Chinese. Seventy-one children with ALL, 25 children with acute myeloid leukemia (AML), and 132 children with nonmalignancy as control were included. Human GGH cDNAs were prepared and analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR)-denaturing gradient gel electrophoresis. The detected variants were further confirmed by DNA sequencing. Novel variant GGH cDNA was cloned, expressed, and characterized. Allelic frequency of GGH 452C→T polymorphism was determined as 9.2% (CT, 11/71; TT, 1/71) in the ALL group, 8.0% (CT, 4/25; TT, 0/25) in the AML group, and 9.1% (TT, 4/132; T/C, 16/132) in the controls, respectively. The total allelic frequency in the Chinese population (9.0%) was higher than those reported in Japanese (5.6%) and African Americans (4.4%), and was similar to Caucasians (10.0%). Association of 452TT+TC with increased rate of hepatotoxicity and mucositis was observed in the ALL patients. Two novel mutations were determined in the coding region of GGH in 2 boys with ALL: one of the mutations was a double nonsynonymous heterozygote (841AG+845AG, K257E/N258S), the other was a nonsynonymous heterozygote (797AG, K242R). The corresponding double-mutant protein showed unchanged enzymatic activity by functional analysis. Allele frequency of GGH C452T polymorphism is determined for the first time among Chinese. A novel double nonsynonymous mutation of GGH was identified in a boy with ALL.