The antibody to hepatitis C virus (anti-HCV) was measured by an immunoassay in 507 serum samples from 94 patients with acute and chronic post-transfusion non-A, non-B hepatitis (NANB) and in 436 healthy blood donors. Anti-HCV was found in 70.8 of patients with acute hepatitis, in 78.2 with chronic hepatitis, and in 1.4 of healthy blood donors. In acute hepatitis, anti-HCV appeared in the serum from 4 to 34 weeks after transfusion and from 1 to 30 weeks after the onset of the overt disease. Three patients with resolving hepatitis (21%) and 2 who developed chronic hepatitis (10%) lost anti-HCV during a mean follow-up period of 28 months. Among the 36 patients with chronic hepatitis, 2 (6%) lost anti-HCV after 12 months and 8 years respectively. These data indicate that in recent years HCV has been the major etiologic agent of acute and chronic transfusion-associated hepatitis (TAH) in our geographical area. The late appearance of anti-HCV from the onset of clinical and biochemical signs of acute hepatitis in more than 70% of patients limits the diagnostic utility of this assay for an earlier serological diagnosis of acute NANB hepatitis. Additional studies are required to determine the diagnostic significance of this antibody in chronic NANB hepatitis.