A 9-year-old boy developed acute renal failure following intravenous acyclovir (30 mg/kg per day) administered for 6 days to treat herpetic encephalitis. Physical findings and urine output were normal, except for increasing blood urea nitrogen (BUN), serum creatinine and mild proteinuria. Acyclovir was discontinued. However BUN and serum creatinine continued to increase and peaked on the following day at 8.6 mmol/l of urea (24 mg/dl) and 194 mol/l (2.2 mg/ml), respectively. Conservative treatment and hydration were carried out. The kidney function returned to normal within 1 week. The use of acyclovir when necessary in renal failure patients is discussed.