In 32 patients with long-term contact lens wear (up to 19 years), deep whitish opacities directly adjacent to Descemet's membrane were seen in the central part of the cornea. These opacities were seen in soft hydroxyethylmethacrylate (HEMA) as well as in hard (polymethylmethacrylate, PMMA) contact lens wear. These conditions could reduce visual acuity. When contact lens wear was discontinued or when the HEMA or PMMA lenses were replaced by gas-permeable rigid lenses, the lesions gradually diminished and resolved completely in most patients. One possible cause of these opacities is an allergic reaction to thimerosal. Another possible cause is chronic anoxia of the corneal stroma and endothelium. Endothelial cell density was not abnormal, but there was a marked polymegethism of the endothelium as a sign of endothelial stress.