We studied twenty-five skin biopsies from twenty-one patients with acquired immunodeficiency syndrome or its prodrome who presented with the "seborrheic dermatitis" associated with acquired immunodeficiency syndrome and compared them with an equal number of biopsies from patients with seborrheic dermatitis who had neither acquired immunodeficiency syndrome nor its risk factors. We found that although the eruption in acquired immunodeficiency syndrome closely resembles seborrheic dermatitis, histologically it is distinctive because it shows spotty keratinocytic necrosis, leukoexocytosis, and a superficial perivascular infiltrate of plasma cells and, frequently, neutrophils that show occasional leukocytoclasis. In long-standing lesions, hyperkeratosis is often present. None of these histologic features are commonly encountered in seborrheic dermatitis, psoriasis, or other eruptions known to us.