A group of 30 potroom workers who developed an asthma-like syndrome during their work in the electrolytic extraction of aluminium (Alu-Swiss process with prebacked anodes) was followed up after stopping exposure. Subjective respiratory complaints were registered and a non-specific bronchial reactivity test using methacholine was performed once or twice during the follow up period (1984-9). After transfer to other work most workers either had an improvement in respiratory symptoms (60%) or even normalisation (27%); only 13% did not show changes. Bronchial hyperreactivity remained unchanged in 67% of workers, was less pronounced in 13%, normalised in 13%, and deteriorated in 7%. Smoking habits, atopy, and duration of exposure did not seem to have influenced the results obtained. No significant correlation was found between duration of exposure and the speed of onset of respiratory symptoms and the later prognosis of the syndrome. Based on the information collected during the period of observation it appears that reactive airway dysfunction syndrome, once induced, has a tendency to persist. An improvement in subjective complaints may be expected, however, after stopping exposure.