The role of smoking as potential risk factor, selection factor ("healthy smoker" effect) and modifying factor (severity) of asthma was studied in the Epidemiological study on the Genetics and Environment of Asthma, bronchial hyperresponsiveness and atopy (EGEA). The analysis involved 200 adult asthmatic cases recruited in chest clinics, 265 nonasthmatic controls and 586 relatives of asthmatics (147 with asthma). Asthma in childhood was not associated with a reduced take-up of smoking (odds ratio (OR)=1.06 in males and 0.98 in females), but smoker asthmatic cases quit more often than controls (OR = 2.20 (95% confidence interval (95% CI) 1.11-4.34) in males and 2.76 (1.19-6.42) in females). Adult onset asthma was unrelated to ever smoking (OR 1.07 in males and 1.02 in females). In asthmatic cases, active smoking was associated with asthma severity. Current smokers, compared to never and exsmokers, had more asthma symptoms, more frequent (> or =1 attack x day(-1)) asthma attacks (OR 2.39 (95% CI 1.06-5.36)) and higher asthma severity scores. No clear pattern regarding the relationships of smoking habits with asthma was observed in first degree relatives. It is concluded that active smoking is not a risk factor for asthma in adulthood, but that smoking increases asthma severity.