Procedures (methods): Eligible subjects with documented SCD (HbSS, HbS-β 0-thalassemia or HbS-β+-thalassemia) followed at the University of North Carolina (UNC) Comprehensive Sickle Cell Program will be evaluated in this single-center, prospective, cross-sectional study. The patients will be screened for eligibility at the time of a routine sickle cell clinic visit. Patients' data will be obtained in person at the time of evaluation and through review of their medical records. Investigators will obtain information on SCD-related clinical complications and obtain an estimate of the number of lifetime RBC transfusions. Blood samples will be obtained for laboratory tests. Plasma samples for hepcidin, growth differentiation factor 15 (GDF -15), and high-sensitivity CRP will be stored at -80 degrees Celsius until analysis. Other routine laboratory studies including complete blood count (CBC) with differential and reticulocyte count, serum iron profile and ferritin, and liver function tests will be performed at the clinical laboratories of UNC Hospitals.The subjects will have 30 ml. of blood drawn for this research study. Females of child bearing potential will have a urine pregnancy test at the time of the study.