['NUDT15 R139C (rs116855232) is a recently identified genetic factor responsible for thiopurine-induced leukocytopenia and hair loss. In this study, we investigated the association of NUDT15 R139C with 6-thioguanine nucleotide (6-TGN) levels and thiopurine-induced leukocytopenia in Japanese patients with inflammatory bowel disease (IBD).', 'Two hundred and sixty-four subjects (103 healthy volunteers and 161 IBD patients treated with thiopurines) were enrolled. Genotyping for NUDT15 R139C was performed using Custom TaqMan® SNP genotyping assays.', 'The NUDT15 C/C, C/T, and T/T genotypes were 80.7, 18.2, and 1.1 %, respectively. The allelic frequency was 10.2 %. Among 161 IBD patients, there was no significant difference in 6-TGN levels among the NUDT15 genotypes. Forty-five patients (27.9 %) developed leukocytopenia (WBC <3000/μl), and the C/T and T/T genotypes were significantly associated with the development of leukocytopenia (P = 1.7 × 10(-5)). In these patients, 6-TGN levels were not significantly different between NUDT15 genotypes. NUDT15 R139C was significantly associated with early (<8 weeks) (P = 1.03 × 10(-4)) and late (>8 weeks) leukocytopenia (P = 4.3 × 10(-4)). The decrease in WBC count at 2 and 4 weeks was significantly higher in patients with the C/T or T/T genotypes as compared to the patients with the C/C genotype. All patients with the T/T genotype (n = 2) developed early severe hair loss and severe leukocytopenia (<1000/μl). The logistic regression analysis revealed that NUDT15 R139C was the sole genetic factor responsible for the thiopurine-induced leukocytopenia (P = 0.001).'] Is nUDT15 R139C-related thiopurine leukocytopenia mediated by 6-thioguanine nucleotide-independent mechanism in Japanese patients with inflammatory bowel disease?