Such nonpercutaneous routes of hepatitis C virus (HCV) transmission as sexual and perinatal spread are relatively inefficient. Several observations have been cited to support a role for sexual transmission of hepatitis C. Approximately 10% of persons with reported cases of acute hepatitis C in the United States report a history of potential sexual exposure. Anecdotal cases of sexual transmission have been reported, and HCV nucleotide sequence homology has been observed in viral isolates from sexual partners. Similarly, the prevalence of HCV infection is increased in groups with a high risk of exposure to sexually transmitted viral infections. Other observations, however, weigh against sexual transmission of HCV infection. Sexual transmission is negligible in sex-partner studies; alternative risk factors account for many cases of apparent sexual transmission between sexual partners; the prevalence of HCV infection in high-risk groups is much lower than that of other sexually transmitted infections; and the risk of apparently sexually transmitted HCV infection does not always correlate with intensity and duration of sexual exposure. The United States Public Health Service has estimated that the risk of sexual transmission is approximately 5%, well below the risk of sexual transmission of hepatitis B or human immunodeficiency virus (HIV). Similarly, perinatal HCV infection, though documented to occur, is unusual, except in babies born to mothers with very high levels of HCV RNA, including mothers with concomitant HIV infection. Weighing many, often conflicting reports, the United States Public Health Service has estimated that the likelihood of perinatal infection is low, on the order of 5% to 6%, and that breast feeding does not increase the risk of HCV infection in infants of mothers with hepatitis C. Current data do not support household exposure as a risk for HCV infection.