The sequence of alterations occurring in recurrent erythema multiforme was studied with clinical observations, 1-micrometer tissue sections, and immunofluorescence techniques. Lesions evolved through 3 stages: an initial red papule, a vesicle surmounting a red papule, and a target (iris) lesion. Focal endothelial cell swelling was present in clinically normal skin. In the red papule, endothelial cytoplasmic swelling, vacuolization, and nuclear hypertrophy with luminal obliteration of superficial venules developed. These venular alterations were more marked with endothelial cell necrosis, and involved deeper venules as well in vesicular and target lesions. Lymphocytes surrounded the venules and infiltrated the lower epidermis in the red papule and the vesicular lesions. Venular damage was correlated with the degree of infiltration by lymphocytes, apparently the primary effector cell, suggesting the venule as a primary target of injury. Hypogranulated basophils were noted around venules in vesicular and target lesions. Fibrin deposits were identified within and interstitially beneath the vesicles. The presence of lymphocytes, basophils, and interstitial fibrin deposition is similar to the changes of cutaneous delayed-type hypersensitivity and suggests a role for cell-mediated immunity in the pathogenesis of erythema multiforme.