The importance of antigen identification is to increase the pretest probability of hypersensitivity pneumonitis (HP) diagnosis, to increase diagnostic reliability, and to improve prognosis. Antigen identification in clinical practice is primarily implemented by using the antigen questionnaire as a screening tool to pick up a wide range of causative antigens, and the suspected antigens are scrutinized by other comprehensive studies. A proposed antigen questionnaire is designed to comprehensively evaluate frequent causative antigens, which reflect epidemiological data in Japan. The evaluation criterion for determining antigen reliability using the antigen exposure assessment form is proposed. Environmental surveys, although their rationale and methodology are not yet established, complement environmental exposures that cannot be obtained by interview alone.