The iridocorneal-endothelial syndrome (ICE syndrome) is characterised by corneal failure, glaucoma and iris destruction. Specular photomicroscopical and histological studies of the corneal endothelium in this disease show a population of abnormal cells named 'ICE-cells'. In many patients some areas of the endothelium are occupied by ICE-cells and others by normal cells, an appearance described as 'subtotal-ICE'. Specular photomicroscopical observations suggest that ICE-cells and normal endothelial cells may actively interact at the boundary zone where they meet. The purpose of this study was to examine the ultrastructural appearances of the boundary zone to gain insight into the cellular pathology of this region. Thirty-five corneas taken from patients with the ICE syndrome were examined by light, transmission and scanning electron microscopy. The subtotal-ICE appearance was demonstrated in four specimens. The morphology of ICE-cells at the boundary zone suggests that they are non-motile but also implies a general state of high metabolic activity. Many of the normal endothelial cells in this region are damaged, an appearance which may result from a toxic effect from the nearby ICE-cells.