Hepatitis C virus (HCV) transmission has decreased with the adoption of universal blood donor screening and social policies to reduce the risk of infection in intravenous drug users, but remains a worldwide health problem. The objective of this study was to evaluate the phylogenetic relationships among sequences from different HCV genomic regions from sexual partners of infected patients. Nine couples with a stable relationship and without other risk factors for HCV infection and 42 control patients were selected, and the NS3 and NS5B regions were analysed. Phylogenetic analysis showed that viruses from five of the couples had a common origin, clustering in the same monophyletic group, with bootstrap values greater than 70. For the other couples, monophyletic groups were observed, but without bootstrap support. Thus, using two different viral genome regions, a common source of infection was observed in both members of five couples. These data strongly support HCV transmission within couples.Hepatitis C virus (HCV) infection constitutes a major health problem throughout the world and is related to severe liver cirrhosis, end-stage liver disease and hepatocellular carcinoma (Chevaliez & Pawlotsky, 2007).The only member of the genus Hepacivirus of the family Flaviviridae, HCV is an enveloped virus with a positivesense, single-stranded RNA genome of approximately 9.6 kb, encoding a single polyprotein of approximately 3000 aa that is cleaved into structural and non-structural proteins. HCV is classified into six genotypes, HCV-1 to -6, with each genotype further subdivided into subtypes, such as HCV-1a and -1b (Simmonds, 2004).Although intrafamilial transmission is not well documented, several studies have reported sexual and vertical transmission and close contacts as sources of contamination (Cavalheiro, 2004;Cavalheiro et al., 2009;Chiaramonte et al., 1996; Keiserman et al., 2003;Mastromatteo et al., 2001). Sexual transmission is not well determined, although studies have indicated a low effectiveness of infection via this route (Akahane et al., 1994). The estimated risk for sexual transmission is 0-0.6 % year 21 for partners who maintain monogamous relationships for a long period of time, and 1 % year 21 for those with multiple partners (Terrault, 2002). Some studies show high serum anti-HCV positivity rates among sex professionals, men who have sex with other men, and healthcare professionals (Nakashima et al., 1992;Ndimbie et al., 1996;Thomas et al., 1995). If the index case is co-infected with human immunodeficiency virus (HIV), the sexual transmission risk is higher (Filippini et al., 2001).As with sexual and vertical transmission, occupational exposure has been documented but is rare (Alter, 2002; Alter et al., 1999). At least 10 % of HCV-infected patients do not report exposure to any of the known risk factors, and these are considered sporadic cases. Detailed molecular analysis may be able to establish the common ancestry of viruses circulating in a family and to support the hypothesis that close familial...

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