Nitric Oxide and Sickle Cell Pain

Background: \- Sickle cell disease often causes crises, with episodes of pain. Many people with sickle cell disease also have pain between crises. Inflammation is an important part of sickle cell pain. It may be related to levels of nitric oxide. Nitric oxide is a gas in the body that helps relax blood vessels and may be related to the pain from sickle cell disease. Researchers want to study the relationship between blood levels of nitric oxide and pain in people with sickle cell disease. Researchers also want to study how certain genes express themselves related to sickle cell pain. Objectives: \- To collect blood samples and other genetic expression information to study sickle cell pain and its relation to nitric oxide levels in the blood. Eligibility: * People at least 18 years of age who have sickle cell disease. * Healthy volunteers at least 18 years of age. Design: * This study requires a screening visit and four study visits scheduled 1 week apart. Each visit will last about 1 hour. * Participants will be screened with a medical history and physical exam. They will complete questionnaires about pain levels (if any). They will also provide blood samples for genetic and other testing. * Participants will have a breath test to see how much nitric oxide they exhale. They will also have a test of their ability to detect small changes in temperature and touch. * Participants will keep a diary to record daily pain levels and pain medicines taken. They will write down what they eat to track foods that contain nitrates (such as meats like ham and bacon and vegetables like beets and spinach). * At each of the four study visit, participants will bring the pain diary, provide blood samples, and have breath nitric oxide tests.