In a previous report, we described autoantibodies against the desmosomal plaque proteins desmoplakin I and II (dp I and II) in patients with erythema multiforme (EM) major. In the present study we investigated ten EM major and eight EM minor patients for circulating autoantibodies and performed clinical and immunomorphological evaluations. Seven out of ten EM major patients revealed anti-dp I and II autoantibodies. Antigens were biochemically characterized by Western blotting and immunoprecipitation of epithelial-cell-derived protein extracts. These autoantibodies bind in vivo to lesional skin/mucosa in a pemphigus-type dotted pattern along the cytoplasmic membranes of keratinocytes. Ultrastructural immunolocalization studies confine in vivo bound autoantibodies to the cytoplasmic desmosomal plaque. Autoantibody binding studies with the sera of such patients demonstrate that the target antigens are not restricted to squamous epithelia but are also expressed in simple and transitional epithelia, on hepatocytes, and on cells of mesenchymal origin, e.g., myocardial cells. Comparing the clinicopathological features of ten patients with EM major, we could not define any discriminating clinical symptoms among patients with or without autoantibodies. Histopathological examination, however, revealed that only patients with EM major and autoantibodies against dp I and II show suprabasal acantholysis in lesional skin and mucous membranes, suggesting a potential role of the humoral immune response in the pathogenesis of this disease. These findings suggest that these autoantibodies define a subset of patients within the clinical spectrum of EM.