The kinetics of IgM antibodies to hepatitis A virus (HAV) following an acute infection, were studied in 17 children. Antibodies disappeared in two patterns, one group at 113 +/- 18 days after the acute infection in the children, and the second group at 283 +/- 90 days. The same two patterns of kinetics were seen in adults. We conclude that IgM anti-HAV antibodies can be found in the sera of children for over 6 months after the acute infection, as was observed in adults.