Hepatitis C virus is a pathogen causing severe hepatic diseases. Though HCV genotype 6a circulated prevalently in Hong Kong, its sequence information is greatly in deficient. The molecular characteristics and epidemiology of HCV 6a were thus extensively investigated in this thesis. The distribution of HCV genotypes in Hong Kong was analyzed from 1055 samples by the Ohno's method. HCV 6a accounted for 23.6% HCV infections in the general population and 58.5% in the injecting drug-users. It is prevalent in Hong Kong, associated with younger age, injecting drug-user status and the male gender. Fourteen independent HCV 6a isolates were sequenced for their full-genomes. They share a sequence homology of 94.5% between each other. HCV 6a had undergone high frequency of recombination. Four (28.5%) of 14 isolates were found having recombination with other strains in different genomic regions. Evolutionary pressure on HCV 6a genomes was analyzed by evaluating dS/dN ratio. NS4A, NS4B and NS3 showed higher dS/dN ratios (16.94-29.30) indicating a purification effect, whilst NS2, E2 and p7 showed lower dS/dN ratios (2.35-7.33) indicating a positive selection effect. This pattern of evolutionary pressure distribution alongside genomic regions was similar to the observations in HCV 1b. However HCV 6a eISDR experienced less extent of positive selection than HCV 1b eISDR (dS/dN = 12.82 vs 4.96) did. Evolutionary history of HCV 6a was inferred by Bayesian coalescent analysis. Twenty-six heterochronic, 513-bp HCV 6a partial-NS5A sequences and 63 HCV 1b sequences were analyzed. The time of exponential growth of HCV 6a in Hong Kong was postulated as during 1986 to 1994, overlaps with the time 1987 to 1997 when the second Vietnamese Boat People influx event occurred. Rooted phylogenetic analysis showed that Vietnamese HCV 6a strains were ancestors of the Hong Kong strains. Hence, a hypothesis was raised that HCV 6a outbreak in Hong Kong may be related to the Vietnamese Boat People influx event. The sequence variations within the eISDR of HCV 6a and HCV 1b were explored for correlation with the outcome of IFN-alpha/ribavirin combination treatment. Twenty-five HCV 6a patients and 37 HCV 1b patients were recruited. Three amino acid variations I2160V, V2256I, and I2292V were significantly correlated with the treatment outcome (P < 0.05) for HCV 6a. Three variations R2260H, V2268I and S2278T (P = 0.023-0.076) were weakly correlated with the outcome for HCV 1b. None of correlated variations located within the previously defined ISDR. These pieces of information can be helpful for predicting the outcome before the commencement of treatment. / Zhou Xiaoming. / "May 2006." / Adviser: Paul K. S. Chan. / Source: Dissertation Abstracts International, Volume: 69-01, Section: B, page: 0207. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2006. / Includes bibliographical references (p. 178-186). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.