SummaryThe use of intravenous acyclovir can be particularly complicated in pediatric patients with evolving renal impairment, because of intraindividual pharmacokinetic variability linked to the patient's clinical condition. The objective of this study was to use therapeutic drug monitoring data to assess acyclovir intraindividual pharmacokinetic variability during several types of renal replacement therapy. Bayesian adaptive control of acyclovir dosage regimen was performed in a pediatric patient with bone marrow transplant who developed severe renal impairment. Acyclovir pharmacokinetic parameter values corresponding to the different techniques and periods of renal replacement therapy were estimated using USCPACK PC Clinical Programs and therapeutic drug monitoring data. Results showed a wide intraindividual pharmacokinetic variability during CAVH, CAVHDF, and CVVHD, reflecting not only the performance of each dialysis technique but also the difficulty in making use of each one. The acyclovir elimination rate constant was higher during CVVHD compared to CAVH or CAVHDF. Bayesian method appears to be valuable in assessing intraindividual pharmacokinetic variability, as it allows the clinician to deal with sparse routine patient data.