Valacyclovir is an effective oral agent for the treatment of herpes virus infection, however, the pharmacokinetics of the drug are altered in renal failure. It is increasingly recognized that dose adjustment of oral valacyclovir in renal failure is necessary to avoid neurotoxicity. We studied this drug in a continuous ambulatory peritoneal dialysis (CAPD) and immunocompromised patient. She developed neurotoxicity with an adjustment dosage of valacyclovir for a cutaneous zoster infection. The elimination half-time (15 h) was similar to that reported for end-stage renal disease patients, while the steady-state volume of distribution (85 l) and the area under the curve concentration (127 mg/l·h) were greater. The mean CAPD dialysance was only 5.27 ml/min with less than 1% of an administered dose being recovered in the 24-hour dialysate. 48 h after interrupting treatment, she recovered normal neurological status and 500 mg of valacyclovir every 2 days was effective and well tolerated.