Hematoxylin-eosin staining, the TUNEL method for in situ detection of the intranuclear DNA fragmentation, which indicates apoptosis, and electron microscopy were used to study the morphologic changes in specimens from the eyes of 8 patients with secondary glaucoma and 2 normal control eyes to evaluate our hypothesis that apoptosis causes a decrease in retinal ganglion cells in human glaucomatous eyes. The TUNEL method permits identification of intranuclear DNA fragmentation. Apoptosis was found in the ganglion cells of 2 glaucomatous eyes with recent sight loss, and in the ganglion cells of a control eye from a 95-year-old subject, taken at autopsy. Results of our study indicate that a decrease in retinal ganglion cells in glaucomatous eyes is caused by apoptosis. In addition, apoptosis resulting from aging must be considered in order to understand the reduction of retinal ganglion cells in the glaucomatous eyes of elderly patients.