Fourteen asthmatics volunteered to stop smoking. Seven managed to stop for only 24 h and seven for 1 week. There was a significant increase in peak flow and specific airways conductance as early as the first 24 h of abstention, while after 7 days of abstention there was a further increase in peak flow and specific airways conductance and a reduction in bronchial responsiveness to histamine. Two subjects felt that their symptoms were worse while abstaining and one of these showed an increase in bronchial responsiveness. After abstention for 1 week, four of the seven subjects recorded an improvement in symptoms. Asthmatics who smoke should be encouraged to stop. Despite an improvement in symptoms and objective measurements, asthmatics may find it difficult to stop smoking and will need considerable help and encouragement if they are to succeed.