Hepatitis C virus (HCV) genotype and other host and viral factors influence treatment outcome in chronic HCV infection. We evaluated the effect of race and genotype on interferon and ribavirin treatment outcome in 70 Southeast Asian (SEA) and 50 white patients. Genotype was based on the 5' untranslated region (5'UTR) with a commonly used line probe assay (INNO-LiPA HCV II) that may mistype genotype 7, 8, or 9 as 1b. HCV core region sequencing resulted in reclassification of 8 genotype 1 and 25 genotype 1b SEA subjects as genotype 7, 8, or 9. Twenty-six SEA genotype 7, 8, and 9 (79%) and 10 SEA true genotype 1b (59%) patients achieved a sustained virologic response (SVR) compared with 15 (34%) white genotype 1b patients. Logistic regression analysis showed that SEA patients with genotype 7, 8, or 9 were more likely to achieve a SVR than white genotype 1b patients (OR 16.56; 95%CI 4.16, 65.91) as were SEA true genotype 1b patients compared with white genotype 1b patients (OR 4.63; 95%CI 1.19, 18.04). In conclusion, a proportion of SEA patients classified by INNO-LiPA as genotype 1b were in reality genotype 7, 8, or 9. In comparison with white genotype 1b patients, both SEA genotype 1b and SEA genotype 7, 8, and 9 patients showed a significantly greater SVR. HCV core sequencing was necessary to determine genotype accurately in persons potentially exposed to HCV genotypes 7, 8, or 9. This study also supports the concept that race and ethnicity are important determinants of treatment outcome in HCV infected patients.