Respiratory disorders in homozygous drepanocytosis and double SC heterozygosis are mainly dependent on two factors : repeated infections with, more particularly, pneumococcus or mycoplasma, and epidoses of occlusion of the pulmonary circulation. Mutual reinforcement of these two factors occurs, in so far as the relative hypoxia of an infected lung increases the risk of falciform and thrombosis formation in the pulmonary arterioles. The particular physical and chemical properties of the drepanocyte red cell, and the anaemia, themselves lead to parallel disturbances in pulmonary circulation function and gas exchanges. Possible development of respiratory insufficiency is generally, therefore, the result of chronic pulmonary arterial hypertension. However, the results of respiratory function tests are often fairly analogous to those observed in moderate interstitial fibrosis : reduction in vital capacity, alveolo-capillary block, reduction in CO diffusion space, a shunt effect, and diminished pulmonary compliance. This problem is discussed in relation to findings of diffuse interstitial fibrosis in a 34-year-old man with double SC heterozygosis, who had numerous episodes of bone, abdominal, and pulmonary microinfarcts. The presence of a diffuse interstitial pulmonary fibrosis, evoked by radiological criteria and respiratory function tests and confirmed by transbronchial biopsy, no other aetiology being established, raises the question of the possible genesis of the fibrosis from repeated episodes of microvascular occlusion. Many factors may therefore be involved in the formation of fibrosis of this type, not only the pulmonary artery obstruction from the microthrombi but also the macrophagic perivascular inflammatory response related to the presence of foci of infarction and precipitation of pathological intravascular material.