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BACKGROUND--Serial peak expiratory flow rate (PEF) recording has been advocated as a sensitive and specific means of confirming work related asthma. The optimum number of recordings per day to achieve the best between-reader and within-reader reproducibility and sensitivity/specificity ratio compared with the final diagnosis determined by specific inhalation challenges is unknown. METHODS--PEF recording was carried out every two hours in 74 subjects referred for possible occupational asthma. Specific inhalation challenges performed in a hospital laboratory or at the workplace (positive in 33 subjects and negative in 41) were considered the gold standard. The duration of monitoring at work and away from work was at least two weeks each. Graphs of PEF recordings were generated in four different ways: every two hours, four times/day, three times/day, and every morning and evening. The graphs were assessed by three readers in three different centres in a blind manner. Furthermore, one third of each type of graph was read blind by the same reader one week after the initial interpretation. RESULTS--Agreement between the three readers was a little more frequent (82%) in the case of the every two hour readings than for the other types of readings (70% v 77%). Agreement between at least two of the three readers occurred in 73% of positive challenges (sensitivity) and in 78% of negative challenges (specificity) for every two hour readings. The figures varied from 61% to 70% for positive challenges and from 78% to 88% for negative challenges for the other types of readings. Within-subject reproducibility from one reading to the next (one week apart) was excellent (83% to 100%). CONCLUSIONS--Recording PEF every two hours results in a slightly more satisfactory agreement between readers and in concordance in terms of sensitivity/specificity than less frequent PEF readings, although the four times a day assessment is almost as satisfactory.