Obstructive diseases of the respiratory tract caused by allergizing substances at the place of work must be classified as occupational diseases, provided certain conditions are fulfilled. The etiopathogenesis and special features of the clinical picture and diagnosis of occupationally conditioned allergic obstruction of the respiratory tract, previously known as bronchial asthma, are discussed. In contrast to "spontaneous" allergy, the occupational cause of the disease is due to often massive exposure to allergens at the place of work and the frequently aggressive potency of several industrial allergens, these being considered the actual "shaping" factors. The need for early diagnosis, as far as possible even at the initial "precursor" stage and during the manifestation of the so-called "equivalents", such as allergic conjunctivitis and rhinitis, is stressed. Treatment directed at removing the causes includes, fundamentally, prophylaxis against exposure (considering the time during which the allergens remain dormant without triggering an allergy); this obviously involves change of occupation or place of work, whereas specific treatment aimed at producing hyposensitization must remain an exception. The possibilities for prevention, chiefly from the medical point of view, are described.