Acyclovir is an antiviral agent widely used in herpetic infections in children. Although acyclovir is generally well tolerated, severe nephrotoxicity has been reported in some cases. In this report, we present a 16-year-old girl who developed acute renal failure due to acyclovir treatment and who needed repetitive hemodialysis. Renal biopsy was performed in order to confirm the diagnosis. A diagnosis of drug-related acute tubulointerstitial nephritis with focal tubular necrosis was made.