Professor Claudia Morris reveals a new paradigm for the pathogenesis of asthma: the role of the essential amino acid arginine. Although the human body is capable of synthesizing arginine, this amino acid falls to critical levels during periods of stress and catabolism, such as during trauma, sepsis and burns. Maintaining sufficient levels of arginine is key to improving the outcomes for a variety of diseases, including asthma. How can arginine levels fall so rapidly when the body is capable of synthesizing its own supply? This is due to the actions of the enzyme arginase, which converts arginine to ornithine and urea, thereby lowering arginine levels and decreasing the concentrations of nitric oxide (NO), an important signaling molecule.