BACKGROUND Chlamydia pneumoniae is a pathogen usually associated with respiratory pathology. It has been estimated that this agent causes 5 to 15% of all pneumonias but its incidence among hospitalised children with respiratory infections is unknown. OBJECTIVE Characterization of the Chlamydia pneumoniae respiratory infections in hospitalised children. METHODS Retrospective longitudinal study of the Chlamydia pneumoniae respiratory infections in children admitted between January 1999 and June 2001 in the Department of Pediatrics in the Fernando Fonseca Hospital-Portugal. Sex, age, race, socio-economic status, number of scholar siblings, frequency of nursery/school, parents smoking habits, familiar atopy, personal history, clinical presentation, diagnosis, treatment and follow-up were analysed for each child. RESULTS 55 cases were documented, 60% males and 73% whites. The age on admission varied between 17 days and 14 years-old, with a higher incidence below 5 years-old (60%). Twenty-three (42%) of the children were in nursery or in school and 12 (44%) had siblings at school-ages. Parent smoking habits were documented in 21 (38%) and familiar atopy in 21 (38%) children. Respiratory atopy (6 children), gastroesophageal reflux (3), cerebral palsy (1), cleft palate (1) and prematurity (1). Pneumonia was the most frequent clinical presentation (63%), followed by bronchiolitis (20%). The clinical presentation was not specific, with cough (84%), respiratory distress (65%) and fever (58%) being the most frequent signs. The most prevalent radiological pattern was the interstitial (42%). Coinfection by other agents occurred in five cases: S. pneumoniae (2), H. influenzae tipo b (1), P. aeruginosa (1) and tuberculosis (1). Complications were documented in 32 (58%) children: hypoxemia (20), pleural effusion (8), atelectasis (2) e atelectasis and hypoxemia (2). Macrolides were prescribed in 44% cases. CONCLUSIONS This study calls the attention to the fact that Chlamydia pneumoniae infection is an etiology to be considered in children with respiratory infection and hospitalisation criteria. This infection can occur in all ages. Because the clinical presentation is not specific and complications can occur, a high level of suspicion is necessary for its diagnosis.