Genetic risk factors for ticlopidine-induced hepatotoxicity were determined in 22 Japanese patients with ticlopidine-induced hepatotoxicity and 85 Japanese patients who tolerated ticlopidine therapy without experiencing adverse reactions. There was a significant correlation between ticlopidine-induced hepatotoxicity and five human leukocyte antigen (HLA) alleles: HLA-A*3303, HLA-B*4403, HLA-Cw*1403, HLA-DRB1*1302 and HLA-DQB1*0604 (corrected probability (P)-value (Pc)<0.01). In particular HLA-A*3303 was present in 15 (68%) of the 22 patients with ticlopidine-induced hepatotoxicity and in 12 (14%) of the 85 ticlopidine-tolerant patients (odds ratio, 13.04; 95% confidence interval (CI), 4.40-38.59; the corrected P-value (Pc)=1.24 x 10(-5)). HLA-A*3303 was present in 12 (86%) of the 14 patients with ticlopidine-induced cholestatic hepatotoxicity (odds ratio, 36.50; 95% CI, 7.25-183.82, Pc=7.32 x 10(-7)). Ticlopidine-induced severe cholestatic hepatotoxicity occurred more frequently in subjects with HLA-A*3303 and its haplotype in Japanese patients. These findings may explain the high incidence of ticlopidine-induced hepatotoxicity in Japanese patients mediated via an immune-mediated mechanism.