A 5-year-old girl with a kidney transplant developed post-transplant Epstein-Barr virus-induced lymphoproliferative disease. She was treated with acyclovir, alpha-interferon, and gamma globulin. A transplant nephrectomy was performed on day 4 due to acute rejection and she was started on hemodialysis. The acyclovir dose was decreased at this time. However, 6 days following the start of acyclovir she developed progressively worsening neurological symptoms resulting in a coma on day 8. Fourteen days after acyclovir was begun pre- and post-dose serum concentrations were 7.02 microM and 182.5 microM, respectively. Acyclovir was then discontinued and 2 days later the child's neurological status began to improve. We conclude that acyclovir in children with end-stage renal failure may lead to severe and reversible neurotoxicity, despite acyclovir dosage adjustment based on renal impairment.