A child had both a congenital nevus unius lateris and a linear psoriasis of recent onset which first appeared localized to the nevus, and later became generalized. Histology of the psoriatic lesion overlying the nevus revealed the typical features of psoriasis and epidermolytic hyperkeratosis. Psoriasis and nevus have both disappeared with Ro 10.9359 therapy. Five similar cases of psoriasis overlying an epidermal nevus have been found in the literature. The epidermal nevus is a fertile site for the development of psoriatic lesions in a predisposed person. It is an example of the Köbner phenomenon. A wider definition of this phenomenon is suggested. The features of linear psoriasis are studied and we now believe that true linear psoriasis is not a distinct clinical entity and that previously reported cases correspond in fact to an inflammatory linear verrucous epidermal nevus (ILVEN).