Patients 21 years of age and older with coronary artery blockage that limits blood flow to the heart (as shown by coronary angiography within 6 months of entering the study) and causes chest pain that interferes with the ability to carry out daily living activities may be eligible for this study. Candidates will be screened with blood tests, an exercise test, magnetic resonance imaging (MRI), and 24-hour Holter monitoring of heart rate and rhythm. Participants will undergo the following tests and procedures: