Peak expiratory flow rate (PEFR) has been measured hourly from waking to sleeping in 29 workers with respiratory symptoms exposed to the fumes of soft soldering fluxes containing colophony (pine resin). Thirty-nine records of mean length 33 days have been analysed, and the results compared with the occupational history and bronchial provocation testing in the same workers. From plots of daily mean, maximum, and minimum PEFR, recurring physiological patterns of asthma emerge. The most common pattern is for asthma to increase with each successive working day. Some workers have an equivalent deterioration each working day. Regular recovery patterns taking one, two, and three days are described. The combination of a three-day recovery pattern and a late asthmatic reaction on Monday results in Monday being the best day of each week. Assessment of these records has shown them to be specific and sensitive, provided the worker was not taking corticosteroids or sodium cromoglycate during the period of the record and that bronchodilator usage was kept constant on days at home and at work. The results of the PEFR records correlate well with bronchial provocation testing, and provide a suitable alternative to this for the diagnosis of mild to moderate occupational asthma. The records are of particular use for screening symptomatic workers whose symptoms appear unlikely to be related to work.