In summary, controlled allergen and occupational inhalation challenge tests represent a major undertaking with a predictable degree of patient discomfort and a significant risk of dangerous side effects. Tests must be well controlled so the responses can be differentiated from nonspecific (nonallergic) effects. There is probably very limited routine clinical usefulness for allergen inhalation tests. These tests, however, remain valuable for investigations into the pathogenesis of asthma and for studies on new therapeutic agents. Occupational challenge tests may be used for the diagnosis of occupational asthma, although safer more convenient methodologies may become available in the future. Occupational challenges, however, remain the only way to definitively document sensitivity to certain low molecular weight chemicals that cause occupational asthma by a process of sensitization but in which the immunopathogenesis has not been entirely worked out.