Lung cancer arises as a focal transformation of chronically injured epithelium with cigarette smoke as one of its well recognized causes. Apart from oxidants, cigarette smoke contains several precarcinogens, and it is surprising that not every heavy smoker becomes a victim of malignant disease. This points to the interindividual variability in susceptibility to carcinogens and there are several lines of evidence that metabolic factors are involved in such variability. Metabolism of carcinogens and also the subsequent multisteps of carcinogenesis are affected by host factors and governed by the balance between opposite forces, such as metabolic activation and detoxification, formation, and scavenging of radicals and DNA damage and repair. This implies that carcinogenic compounds can initiate tumor growth only in amounts saturating detoxification mechanisms. In this context it is well known that glutathione plays a crucial role in the detoxification of xenobiotics. N-acetylcysteine (NAC), an aminothiol and precursor of intracellular cysteine and glutathione, has been shown not only to be an efficient antidote in acetaminophen poisoning but also to possess important chemopreventive properties. In this article, sites and mechanisms of the therapeutic action of NAC are reviewed with special reference to its chemopreventive characteristics.