The ultrastructure of Descemet's membrane was studied in 24 eyes, at least two eyes from patients in each of the first nine decades of life. Both the anterior banded layer and the posterior nonbanded layer were regular, predictable structures, with no extra layers or significant substructures present. The anterior banded layer was present in the eye of a fetus of five months' gestation. The thickness of the anterior layer was approximately 2 micrometers and similar in specimens from patients of all ages. The thickness of the posterior nonbanded layer of Descemet's membrane increased significantly with age, averaging approximately 2 micrometers at age 10 years and 10 micrometers at age 80 years.