Cornoid lamellation is a constant histologic marker of porokeratosis in its many variants. This histologic appearance, heretofore thought to be pathognomonic of and restricted to porokeratosis alone, has also been found by us in a variety of other conditions. Cornoid lamellation is, therefore, analogous to the histologic findings of focal acantholytic dyskeratosis, epidermolytic hyperkeratosis, and follicular mucinosis as a distinctive histologic reaction pattern that reflects disordered progression of epidermal cells to cornification.