Dermal collagen fiber bundles (DCFB) are the major constructional element in the dermis. Although degenerative alterations of DCFB have been reported in chronologically aged skin, changes in photodamaged skin have not been fully investigated. We report ultrastructural alterations of DCFB, and their relation to skin elasticity using photodamaged human skin and UV-irradiated hairless mouse skin. The degree to which DCFB were intact and closely packed was evaluated and scored blindly. Exposed skin (outer forearm) exhibited marked ultrastructural degeneration. In UV-irradiated hairless mouse skin, the intact ultrastructural appearance of DCFB was gradually lost with increasing UV dosage; however, marked alterations in DCFB ultrastructure were absent in either human inner upper arm (unexposed) skin or nonirradiated age-matched control mouse skin. Skin mechanical properties were measured using a Cutometer SEM 474 suction extensometer, recording Ue* immediate deformation, Uv* viscous deformation, Uf* final deformation, and Ur* immediate contraction, all normalized for skin thickness. Uf*, Ue*, Uv*, and Ur/Uf were significantly decreased in exposed compared with unexposed skin. Significant positive correlations between degenerative alterations of DCFB and the decrease in Uf*, Ue*, and Uv* were seen. Changes of "% area of wrinkles" in UV-irradiated mouse skin was significantly correlated with degenerative changes of DCFB. Based on these results, we confirm observations made by others that chronic photodamage may have more severe effects on degeneration of DCFB than that of chronologic aging alone. Furthermore, degeneration of DCFB as detected ultrastructurally may, by its effect on skin elasticity, result in an increase in the appearance of wrinkles.