The presence of hepatitis C virus RNA was examined in the sera of 32 patients with acute non-A, non-B hepatitis. Hepatitis C virus RNA was extracted from serum, reverse transcribed to cDNA and amplified by two rounds of polymerase chain reaction. Hepatitis C virus RNA was detected in all patients who developed anti-C100-3 as well as 79% who did not. These data suggest that hepatitis C virus accounts for most cases of acute non-A, non-B hepatitis whether anti-C100-3-positive or not. In addition, hepatitis C virus RNA was detected in the earliest serum samples, 9 +/- 11 weeks before anti-C100-3, and thus may be valuable as a diagnostic marker for acute hepatitis C.