A previous study suggested that bronchodilators were more effective in preventing diurnal variation of peak expiratory flow rate (PEFR) in patients with chronic obstructive bronchitis than asthma (Connolly 1979). In a further investigation of 100 patients there was an association between characteristic patterns and the size of the morning dip. After the use of bronchodilators there was a residual morning dip which was large enough to produce a regular pattern in most asthmatics but few bronchitics. Response to bronchodilators during the middle of the day was a poor predictor of both the presence and size of diurnal variation of PEFR and of the efficacy of regular bronchodilator therapy. Regular assessment of PEFR and its response to bronchodilator therapy over a few days is required to assess the extent of potential variation in airway obstruction.