The principal aim of this study was to evaluate serum leptin concentrations and to analyze the interaction between serum leptin levels and C-reactive protein (CRP) levels, hematological parameters before and after antimicrobial therapy in neonates with bacterial septicemia. We studied 16 neonates with bacterial septicemia and 15 controls. Blood samples in neonates with septicemia were collected just before antimicrobial therapy and 2 weeks after treatment. The mean concentration of serum leptin, CRP levels, and immature/total neutrophil (IT) ratio in newborns with septicemia were significantly higher than those of controls at the start. Two weeks after treatment, serum leptin levels in newborns with septicemia had decreased and were similar to those of controls. Although there were positive correlations between serum leptin levels and serum CRP levels and IT ratio in the septicemic group at the start, there were no correlations between serum leptin levels and other hematological parameters. These results suggest that leptin is not only an adipostatic hormone but also a stress-related hormone.