Background and objectiveTroglitazone is a 2,4‐thiazolidinedione antidiabetic agent with insulin‐sensitizing activities. This agent had been used efficiently in a large number of patients but was withdrawn from the market in March 2000 because of its association with idiosyncratic hepatotoxicity. To address the susceptible genetic factors responsible for the hepatotoxicity associated with this agent, we performed a genetic polymorphic analysis by a target gene approach in troglitazone‐treated Japanese patients with type 2 diabetes mellitus.MethodsOne hundred ten patients treated with troglitazone were recruited into this study. The case patients (n = 25) were recruited through medical professionals who had previously reported abnormal increases in the levels of ALT or AST among their patients. The control patients (n = 85) were recruited through physicians prescribing troglitazone. For statistical accuracy, efforts were made to maximize the size of the case group. Genotype analysis was performed in 68 polymorphic sites of 51 candidate genes related to drug metabolism, apoptosis, roduction and elimination of reactive oxygen species, and signal transduction pathways of peroxisome proliferator–activated receptor gamma 2 and insulin.ResultsThe strong correlation with transaminase elevations was observed in the combined glutathione‐S‐transferase GSTT1‐GSTM1 null genotype (odds ratio, 3.692; 95% confidence interval, 1.354‐10.066; P = .008).ConclusionsThe double null mutation of GSTT1 and GSTM1 might influence troglitazone‐associated abnormal increases of liver enzyme levels.Clinical Pharmacology & Therapeutics (2003) 73, 435–455; doi: 10.1016/S0009‐9236(03)00014‐6