Occupational asthma related to work in potlines in an aluminum smelter has been diagnosed on clinical criteria in 57 workers. About half were regular tobacco smokers but atopy was uncommon. There was a wide range in the time for which each had been employed prior to development of symptoms, but the average was about 20 months. Thirty-four showed nonspecific bronchial hyperreactivity to methacholine. At annual reviews over a period of 5 years following transfer to other work at the smelter, the majority improved in symptoms in 1-2 years and bronchial hyperreactivity returned to normal. However, over the subsequent 3 years, deterioration, not limited to tobacco smokers or atopic subjects, has occurred in some subjects.