Part 1: Patients will be hospitalized at the NIH Clinical Center for 5 days. On the first day after admission, an indwelling catheter (short plastic tube) will be placed in an arm vein to avoid multiple needle sticks for blood sampling. Patients will then exercise on a treadmill. Blood samples will be drawn before the exercise test and 2 hours after the test. The next 2 days, patients will receive a daily injection of G-CSF under the skin. Blood samples will be drawn every day for 5 days to monitor the effect of G-CSF on moving cells into the bloodstream. Additional samples will be drawn to determine the effects of G-CSF on the number of stem cells that become heart cells, as well as any effects on blood clotting ability. After discharge for the hospital, patients will return to the clinic 2 weeks after the initiation of G-CSF treatment for a physical examination, blood tests, and a treadmill exercise test. One month after the initiation of treatment, patients will return to the clinic for a physical examination, blood tests, treadmill exercise test, an MRI study, and fitting with a Holter monitor for 24-hour heart monitoring.