Hepatitis A IgM antibody (IgM anti-HAV), detected by commercially available solid-phase radioimmunoassay, is an accepted marker of acute viral hepatitis A infection. However, persistence of this serological marker far beyond the acute illness and immediate convalescent period has been reported. To determine the persistence of IgM anti-HAV following clinically manifest acute hepatitis A infection, 59 patients with this diagnosis were followed prospectively until this marker disappeared or persisted for greater than 60 days. Timed from the onset of jaundice, IgM anti-HAV persisted for less than 30 to greater than 420 days; most patients became seronegative by 120 days. These findings suggest that some patients may become seronegative early in the disease course while others (13.5%) remain positive for prolonged periods greater than 200 days. Awareness of this marked variability is important in the interpretation of IgM anti-HAV as a serologic marker of recent hepatitis A infection.