The characteristics of Epstein-Barr virus (EBV)-associated infectious mononucleosis (IM) in Chinese children are rarely reported. To evaluate the clinical presentations and risk factors for complications of EBV-associated IM in previously healthy children in Taiwan, hospitalized children with the diagnosis of IM due to EBV infection from January 1998 to December 2002 were enrolled. Patients had to fulfill the serologic criteria for the diagnosis of primary EBV infection [viral capsid antigen immunoglobulin M (IgM)-(+), viral capsid antigen IgG-(+), and anti-Epstein-Barr nuclear antigen (EBNA) antibody-(-) with exclusion of other concurrent infections or underlying diseases]. Ninety eight children were eligible, with 79% younger than 5 years old (mean, 4.0 +/- 2.3 years). The male-to-female ratio was 2:1. Nearly all patients suffered from fever (mean duration 10.3 +/- 6.0 days). Cough/rhinorrhea, tonsillopharyngitis, cervical lymphadenopathy and hepatosplenomegaly were found over half of the patients. Atypical lymphocytosis (mean, 12 +/- 13%) and elevated serum aspartate aminotransferase (AST; mean, 167 +/- 183 IU/L) and alanine aminotransferase (mean, 221 +/- 222 IU/L) were the most striking laboratory findings. Various complications, including hematologic, hepatobiliary, central nervous system, and obstructive airway problems occurred in about 20% of patients with significantly prolonged course of hospitalization. All patients recovered uneventfully under supportive and immunomodulating management. Female gender, no signs of tonsillopharyngitis, white blood cell count < or =10,000/mm3 and AST > or =150 IU/L were significant risk factors for the occurrence of complications. Clinicians should monitor such patients closely and give proper treatment to decrease possible morbidity or even mortality should complications occur.