To examine the clinical efficacy of antibacterial chemotherapy for life-threatening pneumonia, we investigated the clinical outcome after a prolonged infusion regimen of meropenem based on a pharmacokinetics/pharmacodynamics (PK/PD) theory. In 42 patients with severe pneumonia, the clinical efficacy of a prolonged (4 h) drip infusion regimen of meropenem (4 h-group; 0.5 g, b.i.d.; 18 patients) was compared with that of a standard (0.5-1 hour) infusion regimen of meropenem (1 h-group; 0.5 g, b.i.d.; 24 patients). Although there was no significant difference in the duration of meropenem-administration and the rate of decrease in CRP between the 4 h-group and 1 h-group, the mortality rate in the 4 h-group (5.6%) was significantly lower (p < 0.05) than that in the 1 h-group (37.5%). In this study, the meropenem-concentration in the blood of each patient and the MIC of meropenem against the causative bacteria in each patient were not determined; however, it was thought that the superior life-saving effect of meropenem in the 4h-group was mainly due to prolongation of the time above MIC realized by the prolonged infusion regimen. These results suggested that prolongation of infusion time of meropenem was usefu for improvement of the clinical efficacy against life-threatening pneumonia.