The present investigation was based on the postulate, first proposed by Reed and Leone in 1970, that porokeratosis lesions represent clonal growth of epidermal cells. Comparative studies using immunocytochemical staining, epidermal cell culture, and non-equilibrium pH gradient (NEpHG) gel electrophoresis analysis of the keratins extracted were carried out on five patients with various types of porokeratosis. Positive type IV collagen staining on the stratum corneum was found in lesional skin specimens of all patients, but not in normal controls. A search for connections between type IV collagen in the basement membrane of epidermis and the skin surface disclosed infrequent intra-epidermal streaking of type IV collagen and one positively stained trans-epidermal acrosyringium. Positive intra-epidermal laminin staining in porokeratosis lesions confirmed the trans-epidermal passage of basement-membrane materials. Epidermal cells cultured from lesional skin showed low plating efficiency, and all colonies exhibited intracytoplasmic vacuole formation and excessive top cell shedding. NEpHG gel electrophoresis of keratins extracted showed that lesional profiles not only contained keratins normally present in glabrous skin, but also possibly K9 and some additional proteins. K9 had been immunolocalized to periductal cells in previous studies. Our findings, taken together, strongly suggest that porokeratosis epidermis consists of epithelial cells with lumen-forming ability.