Tissue from 134 patients with neurodermatitis and prurigo, pseudoepitheliomatous hyperplasia, pemphigus, and urticaria pigmentosa was examined qualitatively for epidermal mast cells. Epidermal mast cells were found in all of the diseases that were studied except dermatitis herpetiformis. Pemphigus vegetans and dermatitis vegetans were frequently associated with the presence of epidermal mast cells. In other pseudoepitheliomatous diseases, such as tuberculosis verrucosa cutis, blastomycosis, and bromoderma, epidermal mast cells were present in many cases. Four of eight patients with acral dermatitis with elevated IgE blood levels had intraepidermal mast cells; the number was much lower in patients with usual neurodermatitis and prurigo nodularis. Only two of ten cases of alopecia mucinosa showed epidermal mast cells. A single epidermal mast cell was found in ten cases of urticaria pigmentosa. Chronic inflammation associated with epidermal cell proliferation appeared to correlate with the presence of epidermal mast cell.