The immune response is critical in determining the outcome of hepatitis C virus (HCV) infection. Interleukin (IL)-18 is a pivotal mediator of the Th1/Th2-driven immune response. Two IL-18 promoter polymorphisms (-607C/A and -137G/C) and their haplotypes were known to affect IL-18 expression. We examined the role played by these polymorphisms in determining HCV clearance or persistence. Genotyping was performed among African American injection drug users with HCV clearance (n = 91) or HCV persistence (n = 182) and among European Americans with hemophilia who were mainly infected through plasma transfusion. Among injection drug users, IL18 -607A (odds ratio [OR], 3.68 [95% confidence interval {CI},1.85-7.34]) and IL18 -137C (OR, 2.33 [95% CI, 1.24-4.36]) were significantly associated with HCV clearance. A haplotype carrying -607A and -137C (OR, 4.53 [95% CI, 1.77-11.6]) was also strongly associated with viral clearance. No association was found among those with hemophilia. These results suggest that IL18 promoter polymorphism may affect the outcome of HCV infection in certain groups.