Inclusion Criteria:  HbSS or HbSβ0 thalassemia, 18 - 65 years old; non-crisis, "steady state" with no acute pain episodes requiring medical contact in preceding 4 weeks; ability to understand the study requirements.  Exclusion Criteria:  pregnant at enrollment; poorly controlled hypertension; long-standing diabetes with suspicion for diabetic nephropathy; connective tissue disease such as systemic lupus erythematosus (SLE); polycystic kidney disease or glomerular disease unrelated to SCD; stem cell transplantation; untreated human immunodeficiency virus (HIV), hepatitis B or C infection; h) history of cancer in last 5 years; i) End-stage renal disease (ESRD) on chronic dialysis; j) prior kidney transplantation.