There is uncertainty regarding the pathogenic nature of cytomegalovirus in cutaneous lesions co-infected with herpes simplex virus. It is widely believed that herpes simplex virus is the main pathogenic factor in such lesions and that cytomegalovirus plays little if any role. There are, however, isolated case reports that describe cytomegalovirus as an important driving pathogen in such lesions. The authors present two human immunodeficiency virus patients who have cytomegalovirus and herpes simplex virus co-infected perigenital ulcers, one of whom improved on valacyclovir, while the other, who was already on valacyclovir for chronic herpes simplex virus suppression, showed no improvement with a single dose of cidofovir. He only showed rapid improvement when treated with valganciclovir. The latter patient underscores the viewpoint that at least in some cases, cytomegalovirus may be an important driving force behind the formation of such lesions. The authors therefore recommend that clinicians be aware of the possible pathogenic role of cytomegalovirus in these ulcers, and, in nonhealing ulcers, use anti-cytomegalovirus agents to prevent the onset of systemic disease. These results warrant further study of the pathogenesis of cytomegalovirus in co-infected herpes simplex virus ulcers.