Patients with HES who are 18 years of age and older may be eligible for this study. Candidates will be screened with a medical history, physical examination, blood tests, electrocardiogram (EKG), echocardiogram (ultrasound examination of the heart), pulmonary (lung) function tests, eye exam and a bone marrow examination to determine if they fall into the subgroup of patients likely to respond to this therapy. For the bone marrow procedure, an area of skin and bone is numbed and a very sharp needle is inserted into the bone to draw out a sample of bone marrow for evaluation under the microscope.