There is evidence that exposure to paracetamol may increase the risk of asthma and its severity. Paracetamol is widely prescribed, especially in children, and therefore an association with asthma would have important implications for its use. The purpose of this study is to determine whether the regular use of paracetamol by subjects with asthma results in an increase in asthma severity. A randomised double-blind placebo-controlled clinical trial will be undertaken in 60 subjects with mild to moderate asthma. The effects of 12 weeks treatment with twice daily paracetamol on the clinical, physiological and immunological measures of asthma control will be assessed. This study has major public health significance and could potentially lead to management approaches that could reduce the prevalence and severity of asthma.