© 2015 Asian Pacific Society of Respirology. Background and objective Many of the pathological consequences in the lung following inhalation of asbestos fibres arise as a consequence of persistent oxidative stress and inflammation. Inflammatory responses can be observed in asymptomatic asbestos-exposed individuals. There are currently no interventions to reduce inflammatory or oxidative responses to asbestos before disease develops. We investigated the effects of oral N-acetylcysteine (NAC) on indicators of inflammation or oxidative stress in asymptomatic people previously exposed to asbestos. Methods A double-blind, randomized, placebo-controlled study was conducted to assess the effectiveness and safety of 1800 mg of NAC given orally over a period of 4 months. This was a proof of principle study. Effectiveness was assessed using indicators of inflammation or oxidation as primary end-points. Serum levels of total combined thiols (cysteine, cysteinylglycine, glutathione and homocysteine) were used to monitor the NAC supplementation. Results Thirty-four subjects were randomly allocated to NAC and 32 to placebo. Serum levels of total combined thiols were similar between the groups after intervention. There were no differences in levels of inflammatory or oxidative stress end-points between the groups. No adverse effects were identified. Conclusions No evidence was found that NAC supplementation replenishes total combined thiols in the blood of healthy subjects with a history of asbestos exposure. There was also no evidence of reduced indicators of inflammation or oxidative stress. Further studies should determine the conditions required to increase levels of total anti-oxidant capacity in the blood and in the lungs of subjects with either asbestos-related diseases or subclinical lung inflammation.