Epstein-Barr virus (EBV)-specific antibody responses were determined in 43 consecutive pediatric patients who had signs and symptoms of inectious mononucleosis (IM) and positive diagnostic tests for mononucleosis (Monspot). Thirty patients gave clear-cut serologic evidence of primary EBV infections; of the remaining 13 patients, seven had no antibodies to EBV in the acute- or convalescent-phase sera and six showed serologic patterns of past EBV infections. Further testing proved that the initial Monospot results were either false-positive or were incorrectly interpreted in all 13 patients with unidentifiable illnesses but in only two of the patients with current EBV infections. The data confirm the occurrence of classical IM in children and show that the disease and the EBV-specific antibody responses can be virtually indistinguishable from adult cases.