Monitoring of biotransformation reactions are becoming increasingly relevant as the impact of xenobiotics on the metabolism is beginning to unfold and correlations are found between the production of certain biotransformation products and the development of certain cancers. The complex systems of detoxification enzymes generally function adequately to minimize potential damage from xenobiotics and metabolites. However, dysfunction may occur when these systems are overloaded or imbalanced. Some studies have suggested an association between the ability of the body to adequately transform toxic xenobiotics and metabolites, and the etiology of various puzzling disease entities, such as chronic fatigue syndrome, fibromyalgia, multiple chemical sensitivities, and cancer. The Biotransformation and Oxidative Stress Status laboratory (BOSS) within the Focus Area for Human Metabolomics at the NWU is currently investigating Phase I and II biotransformation reactions, carnitine metabolism, Phase I/II metabolite ratios (as indication of accumulation of reactive intermediates), hormone biotransformation metabolites, as well as antioxidant capacity and markers of free radical metabolism.