The relationships of pulmonary function with age, duration of asthma and social factors were studied in 630 asthmatics. Pulmonary function was assessed by peak flow rate at routine attendance (actual function). Persistence of obstruction was demonstrated by a trial of corticosteroids where necessary. The best obtainable peak flow rate was recorded (maximum function). Unrelieved potentially reversible obstruction was assessed by actual/maximum function. Age and duration of asthma were directly associated with poor control of reversible wheeze as well as with persistent obstruction. In addition poor control of potentially reversible wheezè was related to lower social class, current cigarette smoking and lack of central heating. Persistent obstruction was related to life time amount smoked, passive smoking and lack of central heating.