Cytotoxic T lymphocyte antigen-4 (CTLA4) suppresses cytotoxic T lymphocyte activity. We examined the associations of CTLA4 single-nucleotide polymorphisms (SNPs) at promoter site -318 and exon-1 site 49 with clearance of hepatitis C virus (HCV) after treatment with combination interferon-alpha plus ribavirin (IFN-alpha+R) therapy in 79 white sustained responders (SRs) and 79 nonresponders (NRs). SRs had higher frequencies of 49G, alone (odds ratio [OR], 2.3; P=.042) and tightly linked with -318C in a haplotype (OR, 2.4; P=.030). Homozygosity for the -318C-49G haplotype was even more frequent among SRs (OR, 5.2; P=.049). Comparably strong associations persisted after multivariable analysis. Relationships were not seen with non-1 genotype viruses (OR, 0.93-1.25; P>.25). Virus load also declined more rapidly in carriers of both 49G (P=.0095) and the -318C-49G haplotype. CTLA4 49G in exon 1 alone and in a haplotype with -318C promoter is associated with sustained IFNalpha+R response in white patients with HCV genotype 1 infection.