Electron microscopy was performed on mouse skin to study the mechanism of the spontaneous involution of hair follicles during catagen. The reduction in follicles size appears to result from cell deletion by apoptosis, a distinct type of cell death with importance in tissue kinetics. These ultrastructural changes have been misinterpreted in the past as autophagic vacuole formation because of the prominent phagocytosis of the apoptotic fragments by adjacent, surviving cells of the hair follicle.