AIM: To predict which chronic hepatitis C patients are likely to be late-responders, we herein investigated the clinical characteristics of null-responders at 36 wk with hepatitis C virus (HCV) genotype Ib and a high viral load during the course of pegylated interferon (Peg-IFN)/ ribavirin ther apy. METHODS: One hundred forty-two patients with genotype Ib HCV and a high viral load were included in this study. Peg-IFNα2b (1.5 μg/kg once a week) and ribavirin (600-1000 mg per day according to body weight) were administered for 48 wk. We def ined nullresponders as the cases that never cleared serum HCV RNA as determined using RT-PCR until 36 wk. Other patients were def ined as responders. We compared the clinical characteristics (age, gender, body mass index, previous treatment) and HCV RNA titer during the therapy between null-responders and responders.RESULTS: The HCV RNA clearance rate was 17.9% (24/134), 46.3% (62/134), 60.6% (86/142), 86.6% (123/142), and 88.0% (125/142) at 4, 8, 12, 24, and 36 wk, respectively. There were 17 patients (12.0%) who were still null-responders at 36 wk. There were no differences in the clinical characteristics between the responders and null-responders except for the titer and decline rates of HCV RNA at 1 wk and 4 wk. The HCV RNA titers at 1 wk and after 4 wk of treatment were significantly higher in the null-responders in comp arison to the responders (P <0.01). The serum HCV RNA titers of the responders decreased by 1.3 log after 1 wk of treatment, and 1.6 log after 4 wk of treatm ent, respectively. On the other hand, the titers of the null responders decreased by only 0.5 log after 1 wk, and 0.7 log after 4 wk of treatment, respectively. The decrease rates of HCV RNA after 1 and 4 wk of treatm ent were signif icantly worse for null responders than for the responders (P <0.01). CONCLUSION: The HCV RNA titer at 1 wk and 4 wk after initiating treatment may be useful for predicting null-responders to Peg-IFNα2b/ribavirin therapy. However, further investigation is needed to determine the optimal time at which the decision to discontinue the Peg-IFNα2b/ribavirin therapy for null-responders can be made. AIM: To predict which chronic hepatitis C patients are likely to be late-responders, we herein investigated the clinical characteristics of null-responders at 36 wk with hepatitis C virus (HCV) genotype Ib and a high viral load during the course of pegylated interferon (Peg-IFN) / ribavirin ther apy. METHODS: One hundred forty-two patients with genotype Ib HCV and a high viral load were included in this study. Peg- IFNα2b (1.5 μg / kg once a week) and ribavirin 1000 mg per day according to body weight) were administered for 48 weeks. We deflated nullresponders as the cases that never cleared serum HCV RNA as determined using RT-PCR until 36 weeks. Other patients were defined as responders. We compared the clinical characteristics (age, gender, body mass index, previous treatment) and HCV RNA titer during the therapy between null-responders and responders.RESULTS: The HCV RNA clearance rate was 17.9% (24/134) , 60.6% (86/142), 86.6% (123/142), and 88.0% (125/142) at 4, 8, 1 There were no differences in the clinical characteristics between the responders and null-responders except for the titer and decline rates of HCV RNA at 1 wk and 4 wk. The HCV RNA titers at 1 wk and after 4 wk of treatment were significantly higher in the null-responders in comp arison to the responders (P <0.01). The serum HCV RNA titers of the on the other hand, the titers of the null responders decreased by only 0.5 log after 1 wk, and 0.7 log after 4 wk of treatment, respectively. The decrease rates of HCV RNA after 1 and 4 wk of treatm ent were signif icantly worse for null responders than for the responders (P <0.01). CONCLUSION: The HCV RNA titer at 1 wk and 4 wk after initiating treatment may be useful for predicting null-responders to Peg-IFNα2b / ribavirin therapy owever, further investigation is needed to determine the optimal time at which the decision to discontinue the Peg-IFNα2b / ribavirin therapy for null-responders can be made.