The evolution, maturation, and regression of lesions of psoriasis were studied histologically. The earliest histologic changes in pinhead-sized macules of psoriasis consist of a superficial perivascular infiltrate of lymphocytes and histiocytes and dilation and tortuosity of the blood vessels in the dermal papillae. Some lymphocytes move upward into the spinous zone of the epidermis and slight intercellular edema develops. Above these slight spongiotic foci, and after the granular layer disappears, the cornified layer becomes compact, and parakeratosis and epidermal hyperplasia develop. Only then do neutropils move through the epidermis into the mounds of parakeratosis. These changes, initially focal and episodic, become more confluent and constant, resulting in formation of clinical plaques. In spontaneously resolving lesions, inflammatory cells disappear first, the other changes next, and tortuosity of blood vessels in the dermal papillae last.