Occupational exposures can cause adult-onset asthma. Early diagnosis and early avoidance of further exposure to causative agent improves the prognosis of occupational asthma. Occupational and primary care health services have an important role in the identification of new cases of occupational asthma. For the diagnosis of occupational asthma, serial peak expiratory flow (PEF) measurements should be performed in an early stage. Although it requires an effort from the patient, high quality recordings offer the best approach to assess the relationship between workplace exposure and respiratory symptoms. Good guidance and performance of serial PEF measurements in primary care is recommended and is worth the effort.