Psoriasis has been accepted as a clinical and histologic entity. The identity of the condition has been conceded. Yet in carefully investigated cases of psoriasis the involvement has eventuated in other conditions, mycosis fungoides, for instance. At times it is difficult, even impossible, to differentiate clinically between psoriasis and certain other scaling dermatoses, such as eczema, the prefungoid stage of mycosis fungoides, syphilis, lupus erythematosus, generalized superficial epitheliomatosis cutis, lichen planus, pyoderma and other conditions. Not infrequently it is equally difficult or impossible to make an unequivocal diagnosis in the laboratory. In many cases of typical psoriasis there are individual lesions that alone do not suggest psoriasis. The object of this investigation is a clinical and microscopic study of borderline cases of psoriasis, atypical psoriatic lesions, eruptions in patients with typical psoriasis and uncommon and rare examples of the disease in an attempt to confirm or refute