We aimed to prospectively evaluate the clinical and bacteriological effects of piperacillin in children with pneumonia. Twenty-eight patients (6 months to 5 years of age) with pneumonia were treated with piperacillin. In the same period, 95 strains of Haemophilus influenzae and 41 strains of Streptococcus pneumoniae were isolated in our department and the minimum inhibitory concentration (MIC) of piperacillin was determined. The clinical efficacy of piperacillin was excellent in 4 cases, good in 23, and fair in 1; the response rate was 96.4% (27/28). Among the isolates from our department, there were 4 strains (9.8%) of penicillin-susceptible S. pneumoniae (PSSP), 32 strains (78.0%) of penicillin-intermediate-resistant S. pneumoniae (PISP), and 5 strains (12.2%) of penicillin-resistant S. pneumoniae (PRSP). Against S. pneumoniae, the MIC50 and MIC90 for piperacillin were 0.5 microg/ml and 2 microg/ml, respectively. Panipenem showed the best results, followed by piperacillin, ampicillin, and flomoxef. Among the isolates from our department, there were 51 strains (53.7%) of beta-lactamase-negative ampicillin-susceptible H. influenzae, 42 strains (44.2%) of beta-lactamase-negative ampicillin-resistant H. influenzae, 1 strain (1.1%) of beta-lactamase-positive ampicillin-resistant H. influenzae, and 1 strain (1.1%) of beta-lactamase-positive amoxicillin-clavulanic acid-resistant H. influenzae. The MIC50 and MIC90 for piperacillin against H. influenzae were 0.0625 microg/ml and 0.125 microg/ml, respectively. Tazobactam/piperacillin and piperacillin showed the best results, followed by panipenem, ampicillin, and flomoxef. Piperacillin proved to be very useful for the treatment of pneumonia in children.