The clinical evaluation of newly developed asthma in an adult should always include consideration of his occupational environment, since an abundance of different exposures, which are known causes of asthma, occur in workplaces. Two types of occupational asthma (OA) are distinguished, by whether they appear after a latency period: 1)Immunological OA, characterised by a latency period, caused by high and low-molecular-weight agents, with or without an IgE mechanism 2) Non-immunological, i.e. irritant induced asthma. The first step of the clinical evaluation is to confirm a diagnosis of asthma. Second step is to find out if there is a temporo-spatial distribution of symptoms and lung function that are indicative of OA. Third step is to determine if the disease at hand is an IgE or a non-IgE mediated disease. Last step is a challenge test that can be either unspecific, in order to assess the responsiveness of the lung, or specific challenge test, especially for the non-IgE mediated OA. The depth of clinical evaluation may vary from a situation in which a classical history confirms the clinical symptoms in e.g. a baker with confirmed allergy towards well-known allergens and a characteristic pattern in serial measurements of lung function, to more elaborate investigations in a situation with no or unknown allergen. In the latter situation, a specific challenge test might be necessary in order to find the offending agent. Finally, challenge tests are important in order to distinguish a causal relation from unspecific hyperresponsiveness in persons with pre-existing asthma. In these situations, extended sick leave and challenge tests can be the only way to find the answer.