Medical Biology 62:71-77, 1984 Oxygen Radicals: A Commonsense Look at Their Nature and Medical Importance B. Halliwell From the Department of Biochemistry, University of London King's College, London, U.K. Abstract: "Oxygen radicals" are now popular subjects for research papers; several hundred are published each year. Many of these pass rapidly into oblivion, joining the great mass of unread scientific literature that clogs library shelves and dilutes important research findings to an increasingly great extent. The basic chemistry of oxygen-derived species was established years ago by radiation chemists (1,6), but "superoxide" is still endowed with miraculous properties by the uninitiated. Demonstration that the action of a disease or toxin in vivo produces increased lipid peroxidation (a currently-popular scientific activity) means nothing more than the fact that its action produces increased lipid peroxidation: it does not automatically follow that the lipid peroxidation causes the damaging effects of the drug or disease. The purpose of this paper is to explain: i) what oxygen radicals are ii) the evidence that oxygen radicals are important in vivo iii) what needs to be done to establish a role for oxygen radicals and lipid peroxidation in human disease.