The role of peak expiratory flow (PEF) monitoring in the diagnosis of occupational asthma has recently been assessed by several studies, which agree that this procedure should always be used to confirm the relationship between symptoms and occupational exposure. Some specific issues should be satisfied: a minimum number of four PEF measurements in a day; several weeks of monitoring in and out of work; and the presence of specific patterns of PEF changes. The sensitivity and specificity of PEF monitoring to detect occupational asthma, in comparison with the specific challenge test in the laboratory (the "gold standard") have been shown to be fairly high, although many authors believe that PEF monitoring cannot substitute for the specific challenge test. Limitations include: the low sensitivity of PEF to detect mild changes in airway calibre with respect to forced expiratory volume in one second (FEV1); the blunting effect of pharmacological treatment; the episodic and irregular exposure to the sensitizer in the workplace; and the compliance and honesty of the subject. Further studies are required to select the best indices of daily and day-to-day variability to be used in the evaluation of PEF changes between work and out-of-work periods.