This patient presented with features suggestive of chronic airflow obstruction; he had a chronic cough and progressive dyspnoea and airways obstruction that was unresponsive to inhaled salbutamol. Serial measurements of peak flow at work initially failed to show any acute reactions to work; there were, however, slightly lower mean daily peak flow readings on workdays compared with restdays (fig 1). This is often seen in workers with the more severe forms of occupational asthma. Substantial recovery was seen during an 11 week period off work but subsequent return to work clearly documented occupational asthma with considerable falls in peak flow related to work exposure. Serial measurements of peak flow showed deterioration in 5/5 periods at work and improvement in 5/5 periods off work. Previous work has shown that provided improvement away from work or deterioration at work is seen in at least 75% of possible weeks, such records have a 100% specificity for the detection of occupational asthma but are relatively insensitive (45%-70%).56 Serial measurements of peak flow are unable to identify the specific cause of a worker's symptom for which bronchial provocation testing is required.