Research Design: Patients being treated with intravenous polymyxin B will be identified at three clinical sites in the USA and one in Singapore. Patients (n = 250) will have blood collected at various times surrounding a dose of polymyxin B between days 1 and 5 of therapy. Development of nephrotoxicity, clinical response, and bacteriological response will be examined. Total and free plasma concentrations of polymyxin B will be determined. Bacterial isolates will be examined for the emergence of polymyxin resistance. The relationships between polymyxin B PK, PD and TD end-points (e.g. clinical and bacteriological responses, development of toxicity and resistance) will be assessed using pharmacometric analyses. Finally, the obtained information will be used to apply Monte Carlo simulation to examine the impact of various patient characteristics and other factors on polymyxin B PK, PD and TD, in order to establish optimal dosage regimens and AFC algorithms for individual critically-ill patients.