Acyclovir is an effective anti-viral agent in the treatment of herpes simplex and varicella-zoster viral infections. The most famous side effects of this drug are significant nephrotoxicity and neurotoxicity. To know the exact incidence of acyclovir-induced acute renal failure, we review the medical records of 241 patients that received intravenous acyclovir (10mg/kg/day, in 3 divided dose) for the treatment of chicken pox or herpes zoster infection from June 1999 to Oct 2000. From these patients, only 113 patients (mean age: 53.42±19.09y/o, male:55,female:58)had checked the renal function before and after the acyclovir treatment. Twenty three patients (mean age: 48.91±19.44 y/o, male:7,female:16)had elevated creatinine levels after acyclovir treatment. The incidence was 20.35%. There is no significant age difference in patients with and without renal function deterioration. However, patient is another risk factor to get renal failure after acyclovir treatment (p<0.05). In conclusion, though acyclovir induced acute renal failure is well known, the incidences is still high now. It seems that the female patient had more risk to get renal function deterioration after acyclovir treatment. Because these patients were cared by non-nephrologist, this problem was usually easily ignored.