The presence of IgM anti-HAV was investigated in 60 patients with acute hepatitis A using two commercialy available enzyme linked immunosorbent assays (Havab-M EIA, Abbott, and Hepanostika anti-HAV IgM, Organon). High levels of anti-HAV IgM were present for only 30 to 60 days after the onset. Low levels of anti-HAV IgM were detected for as long as two years after acute phase. These results suggest that only high levels of anti-HAV IgM have to be considered for the diagnosis of a recent infection by hepatitis A virus.