In an AIDS patient who had repeated successful treatment with acyclovir in his history, erosive herpes perianalis with herpes proctitis appeared, which persisted over several weeks. High-dose intravenous administration on of acyclovir (500-750 mg, 3 x daily, over 7 weeks) did not reveal any beneficial effects: However, almost complete clearing of the lesions occurred within 3 weeks of intravenous administration of Foscarnet (50 mg/kg body wt., 3 x daily). No relapse was seen in a follow-up period of 4 months. HSV type II was isolated by culture from the erosive lesions before treatment, but no virus was found 1 week after application of Foscarnet. The unusual chronic refractory course of a severe HSV type II infection in AIDS suggests the presence of an acyclovir-resistant HSV strain in this case. This is the first observation indicating acyclovir-resistance in the Federal Republic of Germany and a warning against the unlimited use of acyclovir in AIDS patients. Foscarnet may be beneficial in some of these cases.