We present 124 children who had mononucleosis. The patients were selected according to strict clinical features. Twenty (16.1%) of the 124 children were proved to have cytomegalovirus mononucleosis and 104 (83.8%) children had Epstein-Barr virus mononucleosis. The symptoms were similar in both groups. Significant differences were found only for the presence of cervical lymphadenopathy, which was more frequent in the Epstein-Barr group (83.2%) compared with the cytomegalovirus group (75%). Fever was the most frequent symptom in both groups. Cytomegalovirus mononucleosis was significantly more frequent in children younger than 4 years.