Three AIDS patients with severe cutaneous herpes simplex virus (HSV) infection refractory to therapy with acyclovir and foscarnet (2 patients) were treated with a topical preparation of trifluorothymidine (TFT) and interferon-α. Complete healing of lesions occurred in 1 patient; a second had significant regression of the infected area. In the third, the lesion was stabilized twice after application of the preparation and reduced in size after a subsequent treatment. In vitro studies confirmed that isolates from these patients were acyclovir- or acyclovir/foscarnet- resistant. In addition, they revealed strong synergy between TFT and interferon-α for these isolates and for strains with wild-type drug sensitivity profiles. Topical TFT/interferon-α may be of benefit in the therapy of mucocutaneous HSV infections, especially when they are resistant to treatment with systemic antiviral agents.