In the United States, an estimated 3.2 million persons are living with hepatitis C virus (HCV) infection. HCV transmission occurs primarily through percutaneous exposure to blood, and persons who inject drugs are at greatest risk for infection. The role of sexual transmission of HCV has not been well defined. However, reports over the past decade, mainly from Europe, have implicated sexual transmission of HCV among human immunodeficiency virus (HIV)-infected men who have sex with men (MSM). In late 2005, two HIV-infected MSM, each with acute HCV infection that was suspected to have been acquired sexually, were evaluated at Mount Sinai Medical Center in New York City, prompting Mount Sinai to request referrals of similar patients. During 2005--2010, a total of 74 HIV-infected MSM with recently acquired HCV infection and no reported history of injection-drug use were evaluated. To examine the role of sexual transmission, a matched case-control study and viral analysis were conducted. Results from the case-control study showed that high-risk sexual behavior was the most likely mode of transmission among these men. Phylogenetic analyses revealed five clusters of closely related HCV variants, suggesting networks of transmission among these men. The findings underscore the importance of screening HIV-infected MSM for HCV, particularly those engaged in high-risk sexual behavior.