Identification of the beta s-gene-cluster haplotype and alpha-gene status provide a useful tool to improve the possibility for early detection in high-risk SS patients. The DNA polymorphisms of the beta s-gene-cluster modify the clinical course in sickle cell anemia especially as it involves the risk of end-stage organ failure of the kidney, lung, and brain. In both Africa and America, the CAR beta s haplotype increases the risk of developing irreversible complications at an early age. The degree of anemia, the Hb F concentration, and the preservation (or lack thereof) of G gamma Hb F is haplotype dependent and correlates with the overall clinical course of the patient. Further modulation of the clinical course by the coinheritance of alpha-thalassemia-2 tends to decrease the risk of soft tissue organ failure but increases the risk of osteonecrosis. A single individual can be expected to fit into the overall pattern. Some sickle related illness will eventually occur in all patients. In the presence of a Senegal haplotype, the patient's health is better, with the CAR haplotype it is always worse; severity is intermediate in the Benin. These genetic markers can be used to identify the endangered patient before the onset of irreversible major organ failure. The high risk SS patient with a CAR chromosome or one who is homozygous Ben without alpha-thalassemia-2 should be monitored closely for evidence of vasculopathy-induced microinfarction of the brain, kidneys, or lungs. Such a patient needs preventive therapy before suffering a major hemisphere stroke, losing kidney function, or developing cor pulmonale secondary to restrictive lung disease.(ABSTRACT TRUNCATED AT 250 WORDS)