The ultimate cause of the clinical abnormalities associated with changes in oxygen supply and oxygen utilization is the development of abnormal tissue oxygen metabolism. Until now, there has been no satisfactory term to describe abnormal tissue oxygen metabolism. We propose the term "dysoxia" to fill this gap. There are a number of causes of dysoxia. One of the most interesting is that form of dysoxia related to abnormal mitochondrial structure and function. In this group of disorders, there is abnormal tissue oxygen metabolism, although oxygen supply is normal. Another interesting cause of dysoxia is exposure to high oxygen concentrations. High oxygen concentrations are involved in producing abnormal tissue oxygen metabolism under a number of different circumstances. The concept underlying dysoxia provides a unified approach to a large and important group of disorders involving most branches of clinical medicine.