In this ultrastructural study, albino hairless mice were irradiated with long-wavelength ultraviolet (UVA) (340-400 nm) thrice weekly for 32 weeks for a cumulative dose of 8000 J/cm2. Biopsies were taken from these mice, from age-matched unirradiated controls, and from mice irradiated with UVB for 20-30 weeks with a cumulative dose of approximately 6-9 J/cm2. The most striking UVA-induced changes were 1) elastic fiber hyperplasia without evidence of fiber disintegration, 2) a large increase in randomly deposited microfibrils; 3) massive duplication of vascular basement membrane; 4) extensive endothelial cell damage; and 5) collagen fibers with smaller diameters but without apparent damage. By contrast, after UVB, the hyperplastic elastic fibers frequently appeared to be degraded. Microfibrils were only moderately increased and remained in an organized array. Also, unlike with UVA, the epidermal basement membrane was duplicated whereas that of the vessels was mainly spared. Collagen fibers showed evidence of dissolution. Thus, ultrastructural features provide further evidence that skin damage induced by UVA can be dissimilar to that induced by UVB.