Nonsteroidal anti-inflammatory drug (NSAID)–photoinduced DNA damage in human peripheral blood mononuclear cells measured using the alkaline comet assay is presented. Whereas Tiaprofenic Acid–photoinduced DNA damage was promptly induced (i.e. observed at relatively low radiation doses), Ketoprofen-photoinduced DNA damage was delayed. This prompt and delayed effect is observed with UVA (320–400 nm), UVB (290–320 nm) and solar-simulated radiation and is attributed to the different photochemical properties of NSAID. The results from these experiments, carried out in living cells, confirm the speculations of NSAID-photoinduced DNA damage brought up by the many experiments conducted in solution.