Host genetic factors have been reported to influence the natural history of hepatitis C virus (HCV) infection. We examined whether variation in interleukin 10 (IL-10) and tumor necrosis factor alpha (TNF-alpha) genes would predict the likelihood of sustained response to antiviral therapy. Single nucleotide polymorphisms (SNPs) and microsatellites at two loci encoding the cytokines IL-10 and TNF-alpha were determined by polymerase chain reaction (PCR)-based techniques. Their relationship to the outcome of antiviral therapy for chronic HCV infection was studied in 49 white patients who had a virologically sustained response (SR) and in 55 white nonresponders (NR) to a combination of interferon alfa-2b and ribavirin (IFN + R). Several IL-10 variants were more frequent among SRs compared with NRs. Carriage of the -592A or the -819T SNP was associated with SR (odds ratio [OR] = 2.2; P =.016). The -592A/A and the exclusively linked -819T/T genotypes were also associated with SR (OR = 16.6; P =.013 for either). The haplotype consisting of the 108-bp IL-10.R microsatellite and -3575T, -2763C, -1082A, -819T, -592A was also associated with SR (OR = 2.65; P =.01). Stratification for viral genotype, baseline viral RNA concentration, and histologic status identified homozygosity for the haplotype as the principal determinant: all 5 homozygous individuals achieved SR (OR(crude) = 13.7; P =.025; stratified ORs = 1.9-7.0), whereas heterozygotes differed only slightly from wild-type carriers. In contrast, TNF alleles defined by promoter sequences -238G/A and -308G/A were approximately equally distributed among SR and NR. In conclusion, homozygosity for -592A, -819T or the extended haplotype (108bp) - (-2575T) - (-2763C) - (-1082A) - (-819T) - (-592A) is associated with SR to IFN + R.