OBJECTIVE: To study the pathogenic bacteria of lower respiratory tract infection (LRTI), and age and gender distribution and drug resistance of the pathogenic bacteria in children. METHODS: Sputum specimens for bacterial cultures were collected in sterile tubes from all of the children with LRTI who had been admitted to the Children's Hospital of Zhejiang University between August 2001 and July 2002. Antibiotic susceptibility tests were performed using the Vitek system , the Kirby-Bauer diffuse method and the Etest method after bacteria were identified. RESULTS: Among the 4 238 patients with LRTI during the study period, 1 181 patients were bacteria-positive, with a positive rate of 27.9%. Streptococcus pneumoniae (S. pneumoniae) was the most common (222 strains), followed by Haemophilus influenzae (H. influenzae) (216 strains), Klebsiella pneumoniae (K. pneumoniae) (216 strains), Escherichia coil (E. coli) (169 strains) and Staphylococcus aureus (S. aureus) (89 strains). The isolation rate of S. pneumoniae in females was significantly higher than in males (6.2% vs 4.7%; P<0.05). However, the isolation rates of K. pneumoniae and S. aureus in males were higher than in females (5.1% vs 4.1% and 2.5% vs 1.5%, respectively; P<0.05). A higher incidence of LRTI due to S. pneumoniae and H. influenzae was found in the 1-3 years group, while the incidence of LRTI due to K. pneumoniae, E. coli, S. aureus and E. cloacae was higher in patients under 1 year of age. Antibiotic susceptibility tests showed that rates of penicillin-non-susceptible S. pneumoniae, ampicillin resistant H. influenzae, oxacillin-resistant S. aureus and ESBL-positive K. pneumoniae and E. coli were 55.0%, 16.5%, 41.2%, 42.6% and 4.5%, respectively. CONCLUSIONS: S. pneumoniae, H. influenzae, K. pneumoniae, E. coli and S. aureus were common pathogens of LRTI in children. The infection rate varied with age and gender. Antibiotics for treating LRTI should be selected based on the drug susceptibility test.