The Centers for Disease Control conducted intensive surveillance for acute non-A, non-B hepatitis in four sentinel counties over a 7-year period. Testing for antibody to hepatitis C virus was performed with the newly developed enzyme immunoassay. The incidence of non-A, non-B hepatitis remained relatively stable (average, 7.1 cases per 100 000), but there were significant changes in disease transmission patterns. The proportion of patients with a history of blood transfusion declined from 17% to 6%, but the proportion with a history of parenteral drug use increased from 21% to 42%. The proportion of patients with histories of sexual exposure (6%), household exposure (3%), occupational exposure to blood (2%), or hemodialysis (0.6%) did not change over time. Antibody to hepatitis C virus was found in 45% of patients within 6 weeks of onset of illness and in 68% of patients followed up for at least 6 months. Patients with no history of transfusions were just as likely to be positive for antibody to hepatitis C virus as patients with transfusion-associated hepatitis, indicating that hepatitis C virus is the major causative agent of all non-A, non-B hepatitis in the United States.