OBJECTIVE: This study was designed to investigate the prevalence, clinical characteristics and outcome of upper respiratory infection (URI) caused by Mycoplasma pneumoniae (MP) in children. METHODS: Pharyngeal cultures for MP antibody were performed in 960 children with acute URI. The samples were randomly collected from the outpatient room or emergency room (Observed group). Of the Observed group, there were 232 cases under 1 year of age, and the remainder, were between 1-12 years old. The samples from 100 healthy children aged from 6 months to 12 years were used as the Control group. The prevalence of MP infection between the two groups was compared. The clinical manifestations and the outcome between the patients with MP positive and negative were compared. RESULTS: MP antibody was positive in 31.7% (304/960) of the Observed group but only 9.0% (9/100) in the Control group (P<0.05). The URI patients under 1 year of age had a lower positive rate of MP than those over 1 year old (P<0.05). Coughs and tonsillitis were more common (P<0.05) , but catarrh, gastroenteritic symptoms, herpes, and tetter were rare (P<0.01) in URI patients with MP positive compared with those with MP negative. Pneumonia developed in 14.8% of the patients with MP positive but only 7.0% in those with MP negative (P<0.01). CONCLUSIONS: MP is one of the main pathogens of acute URI in children. Acute pharyngotonsillitis symptoms are predominately presented in children with MP infection. MP infection was commonly seen in children over 1 year old and they are prone to develop pneumonia.