Reactive oxygen species (ROS) cause damage to the structure and function of tissues. Therefore tissues have systems that eliminate ROS. Bilirubin is one antioxidant that reacts with ROS to produce oxidative metabolites. Biopyrrins are one of the metabolites, the level of which in urine reflects oxidative stress. They are measured by non-competitive inhibition ELISA that employs anti-bilirubin antibody (24G7) and the results are corrected for the urinary concentration of cereatinine. Some reports suggested that psychological stress increased oxidative stress markers. Urinary biopyrrins were also elevated by speech stress, and the subjective stress score recorded by the speakers correlated with the level. The result suggests that bilirubin might eliminate ROS generated by psychological stress. From the beginning of the study of biopyrrins, their urinary level has been known to be increased by surgical stress. Furthermore, it was significantly higher in a major operation patient group than in a minor one, and correlated with operation duration. Sepsis increased the level in surgical patients. Ischemia-reperfusion elevates ROS and, as a result, biopyrrin production. An increase in urinary biopyrrins was observed in a coronary spastic angina group after a spasm provocation test, and the level in myocardial infarction patients with NYHA (New York Heart Association) classification became higher. Correlation between urinary biopyrrins and plasma B-type natriuretic peptide (BNP) was also reported. Research that determines the structures of biopyrrins and their clinical application are in progress.