More than 1% of the world's population is chronically infected with hepatitis C virus (HCV). HCV infection can result in acute hepatitis, chronic hepatitis, and cirrhosis, which is strongly associated with development of hepatocellular carcinoma. Genetic studies of HCV replication have been hampered by lack of a bona fide infectious molecular clone. Full-length functional clones of HCV complementary DNA were constructed. RNA transcripts from the clones were found to be infectious and to cause disease in chimpanzees after direct intrahepatic inoculation. This work defines the structure of a functional HCV genome RNA and proves that HCV alone is sufficient to cause disease.