Primary liver cancer is a cancer that starts in the liver rather than a cancer that starts elsewhere and spreads to the liver. Most types of primary liver cancer arise from abnormal cells in the liver (hepatocellular carcinoma). The exact cause of the abnormal cells is unclear, but most people with primary liver cancer also have liver scarring (cirrhosis) and some have had chronic viral infections of the liver (hepatitis B or C) for 20 to 40 years or heavy alcohol drinking for several years. The number of people in the United States with a diagnosis of primary liver cancer started increasing in the early 1980s. Reasons for the increase were unclear. Some researchers thought that the increase might be due to the rise in hepatitis C virus infections that occurred during the 1960s and 1970s. These infections were spread by needle sharing among intravenous drug users, transfusion of unscreened blood products, and unsafe sexual practices. If these infections were related to liver cancer, cancer rates might continue to increase for many years, particularly in groups that were exposed to hepatitis C virus in the 1960s and 1970s.