Three women (29, 31, and 38 years old) with the iridocorneal endothelial syndrome had endothelial regions with a normal-appearing cellular mosaic adjacent to endothelium with enlarged, dystrophic cells typical of this syndrome. An abrupt junction was present between the two types of endothelium. The cells in the areas with normal-appearing mosaics were much smaller than normal, with the smallest cells near the regions of enlarged cells. The areas of normal-appearing mosaic gradually decreased in two patients, disappearing entirely from one eye followed for five years; there remained a diffuse, uniform corneal involvement by enlarged, irregular endothelial cells typical of the iridocorneal endothelial syndrome. Concurrently, peripheral anterior synechiae and iris hole formation increased. This partial corneal involvement probably represents an early stage of the syndrome.