We demonstrate for the first time the appearance of acyclovir resistance in serial varicella zoster isolates from a patient treated with acyclovir. We recovered varicella zoster virus three times over a period of 5 months from the skin lesions of this patient with AIDS who was treated with three courses of intravenous acyclovir and prolonged low-dose oral acyclovir. The isolate recovered from a typical zoster lesion before acyclovir, and one obtained from a hyperkeratotic lesion 2 months later, after intravenous and oral acyclovir, were sensitive to acyclovir and produced normal amounts of thymidine kinase. In contrast, virus recovered from lesions 5 months after the onset, when the patient had received repeated courses of acyclovir, was acyclovir-resistant and thymidine-kinase-deficient. Resistance to acyclovir was associated with persistence of lesions which failed to improve with intravenous acyclovir, but was not associated with new lesion formation.