Psoriasis is the prototype of a group of cutaneous disorders (psoriasiform dermatitides) that show psoriasiform epidermal hyperplasia, defined as regular elongation of the rete ridges with preservation of the rete ridge-dermal papillae pattern. Depending on whether the lesion is early or resolving, psoriasiform epidermal changes may be subtle or prominent. Other histologic clues to the diagnosis of psoriasis include more dilated and tortuous papillary blood vessels, neutrophils within the epidermis associated with spongiosis (spongiform pustules), neutrophils beneath the cornified layer (subcorneal pustules), neutrophils within the cornified and parakeratotic horn, hypogranulosis, and more keratinocytic mitotic figures above the basal cell layer.