We conducted a prospective, open-label study to delineate a relationship between exposure and outcomes in 36 patients treated with cefepime. Twenty patients had documented Gram-negative infections. Timed blood and urine samples were obtained at steady state to determine pharmacokinetic and pharmacodynamic parameters. Microbiological success was significantly correlated with the proportion of the dosing interval that cefepime concentrations exceeded 4.3 x MIC. Our results support in vitro data that suggest bactericidal activity of beta-lactams is optimized at concentrations approximately 4 x MIC. These results should be validated by large prospective clinical trials.