To determine whether asthma can cause irreversible airflow obstruction (IAO) 89 subjects with uncomplicated asthma received intensive treatment for four weeks. FEV1 was measured at 0, two and four weeks; pulmonary elasticity and flow resistance was measured in 46 of the subjects at four weeks. Severity of asthma and cigarette consumption were ascertained by a questionnaire. The mean difference between the predicted and highest FEV1 during treatment was 0.29 L (P less than 0.001). The highest FEV1% predicted correlated (P less than 0.001) with the duration and severity of asthma in the entire group, in the 51 nonsmokers, and in the 47 subjects with the adult onset of asthma. Pulmonary resistance, but not elastic recoil, correlated with the duration and severity of asthma (P less than 0.01). The results show that chronic asthma can cause narrowed airways and IAO, and suggest that this may be prevented by improved control of asthma.