Losartan for Diffuse Myocardial Fibrosis in Sickle Cell Disease

Who May Qualify: 1. 6 years old or older 2. Diagnosis of HbSS or Sbeta0-thalassemia 3. Ability to cooperate with and undergo CMR without sedation or anesthesia 4. Ability to cooperate with and undergo echocardiogram without sedation or anesthesia 5. Patients who are on a stable dose of sickle cell disease-modifying therapy: Hydroxyurea, Voxelotor, L-Glutamine, or Crizanlizumab, for 3 months prior to enrollment will be eligible. Who Should NOT Join This Trial: 1. Current chronic transfusion therapy. Patients who received a simple transfusion for an acute event will be eligible 3 months after completion of transfusion 2. SCD genotypes other than specified in inclusion criteria 3. Any contraindication to CMR such as metallic implants 4. Inability to cooperate with CMR or echocardiography imaging 5. Known congenital heart disease 6. Estimated GFR ≤ to 30 mL/min/1.73 m2 by creatinine clearance 7. Pregnant or lactating females or females of child-bearing potential who are unable to use a medically accepted form of contraception throughout the study 8. Treatment with a renin-angiotensin pathway inhibitor during the 2 weeks prior to enrollment 9. Hypersensitivity to angiotensin receptor II blockers 10. Hyperkalemia (K\>5.5 mEq/L) on a non-hemolyzed sample despite low-potassium diet 11. Hepatic dysfunction defined as serum ALT \> 5x the upper normal limit for age 12. Current lithium therapy 13. Chronic daily use of NSAID 14. HIV infection. Always talk to your doctor about whether this trial is right for you.