Acyclovir is an effective, frequently used antiviral agent. Adverse effects of this drug are well known and are especially seen with high doses and/or dehydration. In this article, we report a 6-year-old boy with leukemia with nonoliguric acute renal failure in normal hydration status after using acyclovir treatment. He had no preexisting renal impairment, and there were no additional symptoms. Dimercaptosuccinic acid radionucleid scyntigraphy and other laboratory findings revealed impairment of proximal tubule function, in addition to distal tubule. We emphasize that renal functions should be monitored carefully during treatment with acyclovir, and asymptomatic nephrotoxicity must be kept in mind.