1. This article is concerned with the radiation effects on nail based upon macroscopic as well as histological investigations, and composes a series of studies on “Radiation Effect on Hard Tissues” in our Department of Radiology6-11). 2. The nail plate is produced by the nail mat1ix and classified into two layers, the outer and the inner, of different stratification. The outer layer originates from the posterior half of the nail matrix and, on the other hand, the inner layer from the anterior half. The growth of nail plate is influenced by two factors, i.e., formation of nail plate at matrix and forward march of the plate induced by hyponychium. 3. The length of nail plate on mid longitudinal section is 4.5 mm and length of nail bed is 2 to 3 mm on the same section. 4. Among the nail tissues the radiosensitivity is nail matrix, hyponychium and nail plate in order. In nail matrix, the posterior half is more sensitive than the anterior half. 5. The histological changes produced by a single 1000 R irradiation are slight, but ones in a single 3000 R irradiation are so severe that no recovery from the damage are observed. In a single 2000 R irradiation, the changes are severe but reversible. 6. The effect of irradiation to nail is lowered by fractionation. 7. The shedding of the nail due to irradiation, same as the histological change, is also more prominent in the higher dose and in less frequent fractionation. 8. The shedding of the nail is induced by combined processes such as the isolation of the nail plate from matrix, the separation of nail plate from the nail bed and the forward movement of the nail plate. 9. The recovery factor in nail is about same as one in the skin. 10. The growth of human nail is delayed by irradiation. 11. The radiosensitivity of nail is slightly lower than one of skin
