Exposure of cells and human skin to UVA radiation oxidatively induces damage to DNA that consists mostly of 8-oxo-7,8-dihydroguanine (8-oxoGua), together with relatively minor amounts of oxidized pyrimidine bases and oligonucleotide strand breaks. Singlet oxygen generated by the type II photosensitization mechanism forms 8-oxoGua, with a small contribution by hydroxyl radicals. The mechanisms of UVA-mediated formation of DNA oxidation products indicate the absence of induction of double-strand breaks.