A study of the effect of passive smoking on patients with asthma is presented. Six patients were exposed for one hour to the air in a room in which tobacco smoke was produced mechanically over that period. The effects on symptoms, lung function and airways sensitivity to inhaled histamine were then measured and compared with the same patient's responses during a control day when they inhaled smoke-free air. All six patients developed chest tightness and symptoms similar to an attack of asthma. The findings of respiratory and sensitivity tests suggest: (i) that passive smoking may trigger asthma attacks in subjects who suffer from asthma and (ii) that the airways of such subjects show increased histamine reactivity four hours after the passive smoke exposure.