Clinical resistance to acyclovir of herpes simplex virus infections in immunocompromised patients.

Herpes simplex virus strains, isolated from three immunocompromised patients whose infections showed clinical resistance to acyclovir, were studied as treatment progressed. Virus isolated from two patients remained sensitive to acyclovir throughout. Isolates from the third patient, who had received a prolonged course of oral acyclovir, showed a sharp decrease in drug sensitivity which corresponded to loss of thymidine kinase activity. No changes in restriction endonuclease profiles were observed in isolates from the same patient as treatment with acyclovir progressed.