INTRODUCTION: The relationship between the respiratory tract and the environment is complex and is based on the dynamic interaction between genetically determined host-defence mechanisms and the toxicity of inhaled pollutants. STATE OF ART: Although the World Health Organisation has decreed that smoking is an avoidable cause of premature death, the epidemiological evaluation of the effects of smoking remains problematical. Indeed, basing assessment on duration of habit and quantity smoked may be miscalculating the true burden of the effects of smoking. Smoking, as a risk factor, is all too often perceived to be confined to development of chronic obstructive pulmonary disease. The consequences of passive smoking on children respiratory health have been established. In adults, active smoking has been related to asthma severity. Recent data demonstrate an effect of passive smoking on the incidence of asthma in adults. PERSPECTIVES: Objective markers of smoking are rarely used in epidemiological studies of asthma. CONCLUSIONS: However, the paucity of published studies linking smoking with asthma should not immediately lead to the conclusion that there is not a causal relationship.