Ten cases of porokeratosis (Mibelli) seen during the last year are reviewed. The disease may assume different clinical forms. Lesions may be faint or prominent, small or extensive, atrophic or hyperplastic, hyperpigmented or hypopigmented, asymptomatic or pruritic. They may simulate other dermatoses like lichen planus, verrucae, actinic keratoses, epithelial nevi and carcinoma. The demonstration of the cornoid lamella in histologic sections, however, is diagnostic. The results of histochemical studies showed that the cornoid lamella is composed of parakeratotic epidermal cells. These findings and the occurrence of buccal lesions in one of the patients tend to refute the concept that the eccrine sweat duct unit is the only primary site of involvement in this disease.