Two patients with presumed herpes simplex encephalitis developed severe non-oliguric acute renal failure shortly after acyclovir infusions. Renal function returned to normal in less than 3 weeks after discontinuation of acyclovir. Renal biopsies done during the acute phase demonstrated interstitial oedema, eosinophils and cellular aggregates in both and granulomata in the second case suggesting acyclovir-induced hypersensitivity interstitial nephritis.