In a prospective study 43 consecutive children in hospital, aged between 6 months and 7 years and displaying at least one of the clinical signs of infectious mononucleosis (IM), were investigated for Epstein-Barr (EB) virus-specific IgM antibodies by an indirect immunofluorescence test. On this basis EB virus infection was considered confirmed in 8 patients, each of whom had IgM antibodies in the initial serum sample. In one additional patient, IgM antibodies were only detected in a second sample. The IgM antibodies disappeared with 3-11 weeks. Assessment of IgG antibodies had no diagnostic value in the acute phase of IM. Clinically the 3 youngest children, about 1 year of age, were diagnosed as having pneumonia or hepatitis, the 5 other consecutive patients as having IM. Hepatosplenomegaly was fairly frequently associated with IM, while sore throat, lymphadenopathy, and rash were often signs of other diseases. Only the oldest child had heterophil antibodies. Atypical lymphocytes (greater than 10%) were present in 4 of the 9 IM cases and were seen in children with other diseases as well. Our data stress the importance of measuring EB virus-specific IgM antibodies in order to diagnose IM in early childhood.