The diurnal variation (DV) in peak expiratory flow rate (PEFR) has been studied among 132 grain elevator workers who accomplished three daily measurements of PEFR during three weeks. DV was calculated as the difference between the highest and the lowest PEFR as a percentage of the mean PEFR on each day. For the whole group the median was 5.9%. DV was higher among smokers and among workers with work related pulmonary symptoms. Analysis of variance showed that only age (p = 0.012) and smoking (p = 0.016) had a significant effect on DV. Pulmonary symptoms, total IgE, and duration of occupation had no independent impact on DV, whereas the exposure level of grain dust tended (p = 0.082) to have an independent effect. Twelve workers had an abnormally high DV (greater than 20%), of whom seven showed no signs of obstructive respiratory disease by spirometry. If only a single spirometric test had been performed the tentative diagnosis of bronchial asthma could have been missed in these seven workers.