Twelve physically healthy young adult mental hospital patients died unexpectedly while on prolonged chlorpromazine therapy. Five of them had clinically obvious pigmentation of the exposed skin. Two of these had impairment of vision as well. Autopsies were performed on all 12 patients. Extensive deposits of pigment (exhibiting the physical and histochemical properties of melanin) were present in macrophages in the dermis and throughout the reticuloendothelial system, and in the parenchymal cells of internal organs. The dopa tyrosinase reaction indicated increased melanocyte activity in the epidermis.The possible mechanism of production of this pigment is discussed, and the belief is expressed that the increased melanin production is due, at least partly, to the effect of chlorpromazine on the autonomic nervous system, blocking the production of pigment-lightening factors, of which melatonin is the most important. A short outline of contemplated further investigation is given.