We studied a possible correlation between the dinucleotide microsatellite polymorphism of the interferon alpha/beta receptor-1 gene (IFNAR1) and responsiveness of hepatitis C patients to interferon therapy. The length of GT-repeat (n of (GT)(n)) in the IFNAR1 promoter was determined in 157 patients, of whom 50 were responders and 107 were nonresponders to interferon. The genotypes 5/5 (i.e. homozygotes of (GT)(5)) and 5/14 (i.e. heterozygotes of (GT)(5) and (GT)(14)) were more frequently found in responders than in nonresponders: 80 versus 58%, P=0.008. Thus, determining (GT)(n) of the IFNAR1 promoter may help predict treatment outcome in patients treated with interferon alpha and/or beta. In addition, we sought for genetic polymorphism in the promoter region of the interferon alpha/beta receptor-2 gene (IFNAR2), and found single nucleotide polymorphisms (SNPs) at -134 and -75. But these SNPs did not show a clinical significance, as compared with the GT-repeat in IFNAR1, in the context of interferon responsiveness.