Neonates have large inter-individual variability in pharmacokinetic parameters of many drugs due to developmental differences. Theaim of this study was to investigate the factors affecting the pharmacokinetic parameters of drugs, which are commonly used incritically ill neonates. Factors that reflect physiologic maturation such as gestational age, postnatal age, postconceptional age, birthweight, and current body weight were correlated with pharmacokinetic parameters in neonates, especially preterm infants. Comorbiditycharacteristics affecting pharmacokinetics in critically ill neonates were perinatal asphyxia, hypoxic ischemic encephalopathy, patentductus arteriosus (PDA), and renal dysfunction. Administration of indomethacin or ibuprofen in neonates with PDA was associated withthe reduced clearance of renally excreted drugs such as vancomycin and amikacin. Therapeutic hypothermia and extracoporealmembrane oxygenation were influencing factors on pharmacokinetic parameters in critically ill neonates. Dosing adjustment and carefulmonitoring according to the factors affecting pharmacokinetic variability is required for safe and effective pharmacotherapy in neonatalintensive care unit.