The prevalence of antibodies to hepatitis C virus (HCV) was determined in 65 patients with acute non-A, non-B hepatitis (NANBH). The results suggest that HCV is the most common causative agent in posttransfusion NANBH and in drug-related hepatitis. Detection of HCV antibodies does not appear to be a particularly useful diagnostic criterion due to the kinetics of the immune response in the course of the disease.