A differential diagnostic study of dermatologic diseases is bound to concern itself with the similarities of the clinical pictures and nosologic findings in general. The lack of knowledge of the etiologies would make such a study a frustrating experience were it not for the fact that the gathering of clinical findings, laboratory data, follow-up observations, etc., might eventually throw light on the causes of certain diseases. It is our belief that the similarity which exists in the clinical manifestations of seborrheic dermatitis of infancy, erythroderma desquamativa of Leiner, and even psoriasis justifies their classification as erythematosquamous diseases with histopathologic features common to all three. It also points to the clinical pattern of dermatoses due to a disturbance of cell metabolism. On the other hand, we believe that dermatitis exfoliativa of Ritter may be classified as an extreme form of bullous impetigo, with the histologic pattern of an