Extrinsic Allergic Alveolitis (EAA) is a disease characterized by an exaggerated immune response to inhaled organic dusts that can result in alveolar, and occasionally bronchiolar, damage. This damage can disappear completely with the elimination of the antigen and/or treatment with anti-inflammatory agents. Nonetheless, an important group of these patients do not improve; the inflammatory reaction persists. With time, this results in the deposit of abnormal connective, tissue with respect to its quantity and quality, in the pulmonary interstitium destroying the normal parenchymal architecture and making gas exchange impossible. The most common cause of this disease is inhalation of pigeon antigen with a 5:1 predominance in women. Pregnancy has been considered as a case of allotransplantation that produces in the mother a relative hyporeactive condition. In modifying the immune response to the fetus, it is also modified to other foreign antigens. In this work, we studied 80 cases of EAA, 67 females and 13 males. Of the 67 women, 11 (17.1%) presented with the disease after delivery. Symptoms began from 5 days to 7 months after delivery. Eight of the 11 women had been in contact with the antigen before the beginning of the pregnancy without showing symptoms until after delivery. In 4 cases, there had been contact with the antigen in other pregnancies, without symptoms. EAA is a disease involving the immune system in which changes can be seen in its development during pregnancy, as is also seen in other immune diseases, such as Lupus Erythematosus, Rheumatoid Arthritis and other connective tissue diseases. During pregnancy, the mother becomes tolerant. There is a decrease in non-self recognition.(ABSTRACT TRUNCATED AT 250 WORDS)