Oral acylovir treatment rarely results in renal dysfunction. In reported cases concerning renal failure due to oral acyclovir, baseline creatinine levels were unknown or impaired or a predisposing factor for nephrotoxicity was present. We report a 78-y-old female with documented normal baseline renal function and no contributing possible nephrotoxic factor, who developed irreversible renal dysfunction after oral acyclovir treatment. We suggest that in geriatric age groups chronic renal insufficiency should be taken into consideration during oral acyclovir therapy.