This review describes the biosynthesis of nitrogen oxide, its relationship with asthmatic inflammation and its possible role in modulating immune response to inhaled allergens. Nitric oxide has been shown to have many actions from neurotransmission to memory. It is produced enzymatically by nitric oxide synthase which exists in several isoforms. The inducible form has been identified in endothelial cells, smooth muscle cells, epithelial cells, macrophages, fibroblasts, and neutrophils. Nitric oxide has been detected in the exhaled air of several animal species including humans, and increased levels are found in patients with inflammatory airways diseases such as asthma and bronchiectasis. As yet the cellular source of exhaled nitric oxide is uncertain. The precise effects of nitric oxide on the airways have yet to be completely determined, but is has been observed that nitric oxide has a selective suppressive effect on the Th1 subset of helper T cells. This has led to the hypothesis that raised levels of nitric oxide may interfere with the balance between Th1 and Th2 cell types leading to a predominantly Th2 type response associated with allergic asthmatic symptoms. Whilst this simple inorganic gas has an important role to play in the aetiology of asthma, the true extent of this role has yet to be determined.