Superoxide dismutases (SOD) are essential enzymes that eliminate superoxide radical (O₂^–) and thus protect cells from damage induced by free radicals. The active O₂^– production and low SOD activity in cancer cells may render the malignant cells highly dependent on SOD for survival and sensitive to inhibition of SOD. Here we report that certain oestrogen derivatives selectively kill human leukaemia cells but not normal lymphocytes. Using complementary DNA microarray and biochemical approaches, we identify SOD as a target of this drug action and show that chemical modifications at the 2-carbon (2-OH, 2-OCH₃) of the derivatives are essential for SOD inhibition and for apoptosis induction. Inhibition of SOD causes accumulation of cellular O₂^– and leads to free-radical-mediated damage to mitochondrial membranes, the release of cytochrome c from mitochondria and apoptosis of the cancer cells. Our results indicate that targeting SOD may be a promising approach to the selective killing of cancer cells, and that mechanism-based combinations of SOD inhibitors with free-radical-producing agents may have clinical applications.