UVA has been implicated in multistage photocarcinogenesis. HPLC analysis revealed that the 7,8-dihydro-8-oxo-2'-deoxyguanosine content of DNA in HL-60 cells increased immediately after UVA irradiation. Pulsed field gel electrophoresis showed that cellular DNA strand breakage was induced at 1 h after UVA irradiation. Flow cytometry showed that mitochondrial membrane potential (delta psi m) decreased time-dependently in cells exposed to UVA. The activity of caspase-3 and DNA ladder formation increased at 2 h after UVA irradiation. These results suggest that UVA irradiation induces oxidative DNA damage, followed by the loss of delta psi m and subsequent activation of caspase-3, resulting in apoptosis.