Background: Psoriasis is a common, scaly erythematous disease of unknown etiology, marked by remissions and exacerbations of unpredictable onset and duration. Among many etiologic factors, psoriatic keratinocyte is found to play the most important role. Objective: The purpose of this study is to evaluate the hypothesis that the mechanism(s) responsible for the abnormal proliferation of psoriatic keratinocytes may be located within the cell themselves. Methods: Human epidermal keratinocytes were isolated from lesion(PL) and from uninvolved skin (PN) with chronic plaque-like psoriasis and from the normal skin(NN). Keratinocytes were passaged onto culture vessels without the feeder layer and maintained with serum free medium. Growth rates were measured in secondary cultures by MTT assay and ultrastructural findings of cell differentiation were evaluated with a transmission electron microscope.(continue...) (Korean J Dermatol 1999;37(2): 206-218)