The natural history of hepatitis C virus infection is still unclear. This disease has a long clinical latency period and very frequently develops into chronic forms (in 70-80% of cases). Diagnosis of chronic HCV infection can only be based on liver biopsy. Histological scores permit rating disease severity and to compare successive biopsy specimens. Certain clinical peculiarities deserve to be underlined, such as the frequent asymptomatic forms with normal serum transaminase. Histologically, the liver can be either normal or the focus of chronic hepatitis. HCV RNA screening by PCR may or may not detect virus C replication. Histologically documented chronic hepatitis carry a 20% risk of developing into cirrhosis in 10 years. The risk of liver cell carcinoma is around 3% per year in cirrhotic patients. That risk justifies systematic surveillance, as is currently applied to cirrhotic patients in France. Immune disorders are frequent during HCV infection: presence of anti-tissue antibodies and especially anti-LKM1 antibody, cryoglobulinaemia, glomerulopathy, thyroid disease, Sjögren's disease, late skin porphyria, lichen planus, haemolytic anaemia, idiopathic thrombocytopenic purpura. The severity of chronic diseases induced by HCV is certainly increased by alcohol intake and all chronic HCV carriers should be advised not to drink any alcohol.