Patients between 10 and 55 years of age with acute or chronic leukemia, myelodysplastic syndrome, or myeloproliferative syndrome may be eligible for this study. Prospective participants and their donors are screened with a medical history and physical examination, blood tests (including a test to match for genetic compatibility), breathing tests, chest and sinus x-rays, and tests of heart function. They also undergo a bone marrow biopsy and aspiration. For this procedure, done under local anesthetic, about a tablespoon of bone marrow is withdrawn through a needle inserted into the hipbone.