The novel nucleoside 5-iodo-4-thio-2'-deoxyuridine (4) was synthesized and fully characterized by IR, NMR, and MS. Its structure was determined by single-crystal X-ray diffraction. Compound 4 absorbs strongly at 346 nm and is minimally toxic to human tumour cells, but its cytotoxicity is substantially enhanced by low dose UVA radiation. The combined use of 4 and UVA offers a promising route to selectively and effectively kill proliferating cells.