Cigarette smoking is a well-known health hazard, probably not least for patients suffering from asthma. This review gives a short overview of the effects of passive and active smoking on the inception and outcome with of longitudinal changes in the lung function and mortality of patients with asthma. Substantial evidence suggests that smoking affects asthma adversely. Exposure to environmental tobacco smoke, especially maternal smoking in children, may be a significant risk factor for asthma. Such exposure in patients with established asthma is not only associated with more severe symptoms, but also with a poorer quality of life, reduced lung function, and increased utilisation of health care including hospital admissions. Active smoking does not appear to be a significant risk factor for asthma, but is associated with a worse outcome with regard to both longitudinal changes in lung function and asthma-related mortality. Based on current knowledge, it is therefore of utmost importance to encourage patients with asthma not to smoke, and accordingly these patients should be given full support in their right to a smoke-free environment.