Sickle cell chronic lung disease (SCLD) is a prime contributor to mortality in young adult patients with sickle cell disease, especially those with sickle cell anemia (SS). Both perfusion and diffusion defects have been demonstrated, with generalized pulmonary fibrosis and disabling restrictive lung failure. We report 28 cases (25 SS, 1 S beta(0) thalassemia, 1 S beta(+) thalassemia and 1 SO-Arab) which began during the second decade of life and which ended in death by the fourth decade, after an ordered progression to pulmonary failure and cor pulmonale. Myocardial hypoxia with multifocal fibrosis and segmental infarction occurred in more than one-third of the cases and sudden death was a frequent final event. We define 4 stages of SCLD, based on pulmonary function tests, chest roentgenograms, blood gases, and noninvasive cardiac studies; each stage is 2 or 3 years in length, until death ensues in Stage 4. Case-control analysis showed that the significant risk factors associated with SCLD are 1) the total number of acute chest syndrome events in an individual before the onset of SCLD, (p = 0.0001), 2) sickle cell crisis marked by chest pain (p = 0.03) and 3) aseptic necrosis (p = 0.005). Temporal clustering of acute chest syndrome episodes frequently heralds the onset of SCLD. The pulmonary arterial bed, which has low oxygen tension and low pressure in a slow-flow system, is ideally suited to facilitate the polymerization of sickle hemoglobin, causing endothelial damage and culminating in an obstructive arteriolar vasculopathy. Identification of the significant risk factors predictive of SCLD can lead to early diagnosis of the disease; this is the only hope for effective intervention therapy.