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Abstract: Background: Major depression is a frequent adverse effect of interferon-alpha (IFN-α) therapy. Although the indoleamine 2,3-dioxygenase (IDO) enzyme seems to be involved in the pathophysiology of IFN-α-induced depression, no pharmacogenetic study has investigated whether variation in the IDO gene modifies vulnerability to this adverse effect. Methods: A cross-sectional study assessing 277 hepatitis C patients recruited in two specialized outpatient clinics of Brazil. They were interviewed with the Mini International Neuropsychiatric Interview (MINI) approximately 1month after the end of IFN-α plus ribavirin therapy. Genomic DNA of individuals was extracted from venous blood. Three IDO single-nucleotide polymorphisms (SNPs) were genotyped (rs3824259; rs10089084 and rs35099072). Results: MINI indicated that 21.3% of the sample met criteria for a major depressive episode during the course of IFN-α therapy. No association with the diagnosis of a major depressive episode during the course of IFN-α therapy was observed genotype or allele-wise (p >0.05). Current major depression and/or current anxiety disorder was significantly associated with IFN-α-related depression (p <0.005). However, gender, age, route of infection, result of the antiviral treatment, past history of substance use disorders, depression or any other psychiatric disorder showed no association with IFN-α-related depression (p >0.05). Conclusions: Our results suggest no influence of the variants in the IDO gene and the diagnosis of interferon-α-related depression in the Brazilian population. Interferon-α-related depression may impose persistent psychopathology on at least 15% of the depressed patients even 2years after antiviral therapy termination. The cross-sectional design is a limitation of our study, predisposing memory bias. Prospective pharmacogenetic studies are warranted to continue investigation of the impact of IDO polymorphisms on the development of IFN-α-induced depression. [Copyright &y& Elsevier]