Summary. This study investigated the clinical, serological and molecular characteristics of coexistence of both immunoglobulin M (IgM) antihepatitis A virus (HAV) and IgM antihepatitis E virus (HEV) in acute viral hepatitis using a prospective, multicentre design. Among a total of 771 symptomatic cases with acute viral hepatitis enrolled in a Korean city from September 2006 to August 2008, coexistence of IgM anti-HAV and IgM anti-HEV was found in 43 patients (A+E group; 6%), while the existence of IgM anti-HAV alone was found in 595 patients (A group; 77%) and that of IgM anti-HEV alone in 14 patients (E group; 2%). Clinical data analysis and measurement of IgM and IgG anti-HEV were performed using two different commercial kits, and HAV RNA and HEV RNA were detected in available serum or stool samples. The clinical features of the A+E group were similar to those of the A group. HAV RNA detection rates in the A+E and A group were similar, while HEV RNA was detected only in the stool samples of the E group, not in the A+E group. Comparative testing of anti-HEV using two different ELISA kits showed markedly discordant results for IgM anti-HEV positivity and consistently low positivity for IgG anti-HEV in the A+E group. Coexistence of IgM anti-HEV measured by the Genelabs ELISA kit in the setting of hepatitis A appears to yield false-positive results in nonendemic areas of HEV infection. Diagnosis of hepatitis E using IgM anti-HEV should be made with caution.