Patients with HCM 18 to 75 years old are screened for this study under protocol 98-H-0102 and this protocol. Screening tests include blood tests, echocardiogram to measure heart thickness, Holter monitor to record heartbeats, treadmill exercise test, and various imaging tests including a thallium scan, radionuclide angiography, magnetic resonance imaging (MRI), and cardiac catheterization to examine heart function and blood supply.