Despite recent biophysical insights into the molecular pathogenesis of the sickle cell syndromes, our understanding of the relationship of these subcellular events to the variable clinical expression of sickle cell disease remains largely speculative. We have sought to develop quantitative ways to clarify disease pathogenesis, as well as to assess severity and progression. We have developed an analytical phthalate ester technique for separating sickle erythrocytes by density (or intracellular hemoglobin concentration) and have calibrated this method against the preparative Stractan method. Using the phthalate method we have now shown that there are at least three cellular processes contributing to red cell heterogeneity and we are investigating the genetic and biochemical processes that account for the appearance of dense cells in individuals with sickle cell anemia. Ocular studies of the patients show striking correlations between the extent of erythrocyte heterogeneity with conjunctival and retinal pathology. In other physiological studies, we have been evaluating the utility of several non-invasive approaches which characterize the consequences of altered microvascular blood flow. Using the technique of laser-Doppler velocimetry, we have found that forearm cutaneous microcirculatory flow undergoes a unique characteristic periodic pattern. Studies of 2-deoxy-18 fluoro-deoxyglucose brain metabolism using positron emission tomography (PET) have demonstrated frequent anterior cerebral pathology where none was previously suspected. Preliminary studies using magnetic resonance imaging (MRI) suggests that this modality is more sensitive than conventional methods in disclosing early changes of aseptic necrosis of the femoral heads. We hope that these cellular and physiological measurements will allow us to understand better the extreme spectrum of disease manifestations, as well as serve as objective means of evaluating response to therapy in sickle cell anemia patients.