Thirty-eight patients with red cedar asthma proved by inhalation provocation test were studied after they had left exposure for more than 6 months. Twenty-seven patients became asymptomatic, with normal lung function (group A). Three patients had persistent chronic bronchitis with a moderate degree of airway obstruction, probably as a result of cigarette smoking (group B1). Eight patients continued to have recurrent attacks of asthma that decreased in severity after cessation of exposure, and their symptoms were probably due to previous exposure (group B2). The effect of breathing helium on maximal expiratory flow at 50 per cent of the vital capacity was studied. All except one patient in group A were responders (change in maximal expiratory flow at 50 per cent of vital capacity greater than 30 per cent). Two patients in group B1 and 2 in group B2 were nonresponders, suggesting obstruction in the small airways. All patients with red cedar asthma demonstrated bronchial hyperreactivity to methacholine to the same extent as patients with nonoccupational asthma. This hyperreactivity persisted after they left exposure, irrespective of symptoms. It is not known at present whether bronchial hyperreactivity is the predisposing factor in occupational asthma or is the result of the disease.