The diagnosis of type A hepatitis is performed mainly by immunoglobulin M (IgM) anti-hepatitis A antibody assay, but it has not been established whether there is a correlation between changes in viremia and the clinical course of type A hepatitis. We examined hepatitis A virus (HAV) RNA in the sera from type A and non-A, non-B, non-C acute hepatitis and analyzed the relation of HAV viremia with alanine aminotransferase (ALT) and IgM-HA levels. Two hundred sera from 38 patients with type A acute hepatitis and 20 patients with non-A, non-B, non-C acute hepatitis were examined for the presence of HAV RNA. HAV RNA was detected by nested reverse transcription-polymerase chain reaction (RT-PCR) with primers located at the 5′ non-translated region of HAV. HAV RNA was detected in 35 of 38 (92%) type A hepatitis patients and in 60 of 156 (38%) serum samples. In contrast, it was detected in none of 44 serum specimens from 20 non-A, non-B, non-C acute hepatitis patients. In type A hepatitis, the mean ALT level in HAV RNA positive specimens was 1,481 ± 2,042 (range, 20-10,370) IU/L and that in HAV RNA negative specimens 186 ± 330 (range, 8-1,698). The positivity of HAV RNA was correlated with the level of transaminase at the time of sample collection. The mean duration from the onset of symptoms to disappearance of HAV RNA was 18 ± 14 days. The mean titer of IgM-HA in HAV RNA positive cases was 5.0 ± 1.4, in negative cases 5.7 ± 1.1, with no statistical difference. Our results indicate that HAV RNA in serum is detectable in the majority of type A hepatitis cases in their early convalescent phase by nested RT-PCR.