Oral administration of tamoxifen, an endocrine therapy for breast cancer, often induces hepatic steatosis (THS, tamoxifen-induced hepatic steatosis) as a complication, which can progress to non-alcoholic steatohepatitis (NASH). The development of this complication is strongly associated with three clinical risk factors; specifically, insulin resistance, central obesity, and hypertriglyceridemia, however a genetic predisposition to THS has yet to be investigated. The aim of this study is to determine whether genetic polymorphism of the P450c17alpha enzyme coded for by the CYP17 gene, responsible for regulating serum estrogen, has an association with THS. After obtaining informed consent from 180 eligible breast cancer patients treated with tamoxifen, DNA was collected and analyzed by restriction fragment length polymorphism assay and classified into alleles defined as A1 and A2. The absence or presence and extent of THS was evaluated by calculating the liver/spleen (L/S) ratio based on Hounsfield units with a CT scanner. Administration of tamoxifen led to THS (L/S ratio <0.9) in 57 (31.7%) of 180 patients while the remaining 123 (68.3%) patients did not develop THS. A significant difference in the distribution of CYP17 genotypes was observed between patients who developed THS and those who did not (P=0.021). A significantly higher frequency of the A2 allele was seen in the THS group (odds ratio, 1.90; 95% confidence interval, 1.21-2.99). Our study provides the first evidence that CYP17 polymorphism participates in the development of THS, and sheds light on the genetic causes of this side effect and genetic differences between tamoxifen-treated individuals.