An association between erythema multiforme and herpes simplex virus infection has been supported by clinical studies and by the detection by immunofluorescence of herpes viral antigen in sera and skin biopsy specimens of patients with erythema multiforme. In rare cases, the virus has also been isolated in cultures of skin biopsy specimens of erythema multiforme. To investigate further the association between erythema multiforme and herpes simplex virus, we used the polymerase chain reaction for herpes simplex virus to examine skin lesions from patients with erythema multiforme. In this study herpes simplex virus DNA was detected in 11 of 31 biopsy specimens of erythema multiforme; six additional cases showed equivocal amplification results, which is suggestive of low amounts of viral DNA. Seven skin and mucosal biopsy specimens with the histologic changes of herpes virus infection served as positive controls: all were positive for herpes simplex virus DNA. Viral DNA was not detected in control biopsy specimens from skin excised for unrelated conditions. These studies support the association of herpes simplex virus in the pathogenesis of some cases of erythema multiforme. The polymerase chain reaction provides a quick and effective method of detecting herpes simplex virus in lesions of herpes-associated erythema multiforme. Furthermore, the polymerase chain reaction may delineate those cases of erythema multiforme that are etiologically related to herpes virus infection and therefore might be treated with acyclovir to prevent recurrence.