Background/Aims: Acute hepatitis C (AHC) has a high chronicity rate of up to 50~84% if it is not treated. Although the good treatment response to pegylated interferon (peg-interferon) therapy has reported, there is not definite guideline to treat of AHC in Korea yet. The aim of our study was to investigate the clinical course and treatment outcome of AHC in single center of Korea. Methods: We performed a retrospective analysis of 35 patients who were diagnosed with AHC during the period from May 1996 to December 2007. The diagnosis of AHC was based on seroconversion to anti-HCV antibody or the clinical and biochemical diagnostic criteria satisfactory to AHC and on the presence of HCV RNA in first serum sample. The spontaneous resolution was defined as loss of HCV RNA in serum for 6-month in untreated group, and in treatment group, the sustained virological response (SVR) was defined as a index of treatment success. Results : Thirteen of thirty-five patients were treated, six of thirty-five were untreated and observed clinical course, and sixteen patients were not followed up after diagnosis. In treatment group, nine of thirteen (69%) acquired SVR, and two of six (33%) showed spontaneous resolution in untreated group. Ten of thirteen treatment patients used conventional interferon, and another three patients used peg-interferon. Conclusion : Compared with untreated group, there was higher SVR rate in treatment group (33% vs. 69%). So early interferon treatment in acute hepatitis C should be considered. Background: This study was conducted to identify predictors of thyroid dysfunction and to determine whether virologic factors or treatment response affect thyroid dysfunction development during peginterferon (pegIFN) therapy in chronic hepatitis C patients. Methods: Sixty chronic hepatitis C patients treated with pegIFN -2a or -2b in combination with ribavirin from 1st July 2004 to 30th July 2008 were included in this study. Treatment responses were evaluated and thyroid functions were assessed every 4 weeks. Results: Seventeen patients (28.3%) experienced thyroid dysfunction during treatment, and that occurred more frequently in women and in patients with a lower body mass index (BMI). The proportion of patients with a high viral load (a serum HCV RNA titer >750,000 IU/mL) was significantly higher in the thyroid dysfunction group rather than in the euthyroid group(88.2% vs. 48.8%, p=0.005). Among patients with HCV genotype 1, the rate of sustained virologic response was lower, and relapse occurred more frequently in the thyroid dysfunction group than in the euthyroid function group during pegIFN-based therapy(SVR, p=0.024; relapse, p=0.017). The female gender and the high viral load were independent predictors of thyroid dysfunction in multivariable analyses (female, OR 9.44, p=0.009; high HCV RNA titer, OR 8.40, p=0.030) . Conclusion: The risk of thyroid dysfunction during pegIFN therapy for chronic hepatitis C was found to be higher for women and for those with a low BMI and a high viral load.