Published studies have described a strong association with a single-nucleotide polymorphism (SNP) in the inosine triphosphate pyrophosphatase (ITPA) gene and ribavirin (RBV)-induced hemolytic anemia in HCV-infected patients receiving pegylated interferon (pegIFN) and RBV. This study sought to evaluate the effect of these polymorphisms on anemia, hemoglobin reduction, HCV kinetics, and treatment outcomes. Sixty-three patients coinfected with HIV and HCV and 58 patients infected with HCV only were treated with pegIFN/RBV were genotyped using the ABI TaqMan allelic discrimination kit for the 2 ITPA SNP variants rs1127354 and rs7270101. A composite variable of ITPA deficiency using both SNPs was created as previously reported. Statistical analysis was performed using Mann-Whitney test or Chi square/Fishers exact test for categorical data and mixed model analysis for multiple variables. Thirty-five patients (30%) were predicted to have reduced ITPA activity. ITPA deficiency was found to be protective against the development of hemoglobin reduction >3 g/dl over the course of treatment. The rates of hemoglobin reduction >3 g/dl decreased in correlation with the severity of ITPA deficiency. ITPA deficiency was associated with slower hemoglobin decline early in treatment (week 4, P = 0.020) and rapid virologic response (RVR) at week 4 (P = 0.017) in patients coinfected with HIV and HCV. ITPA polymorphisms are associated with hemoglobin decline and in patients coinfected with HIV and HCV it is also associated with early virologic outcomes. Determination of ITPA polymorphisms may allow prediction of RBV-induced anemia and earlier initiation of supportive care to ensure optimal therapeutic outcomes. J. Med. Virol. 84: 1106–1114, 2012. Published 2012. This is a US Government work and as such is in the public domain in the United States of America.