Inclusion Criteria

DISEASE CHARACTERISTICS:

- Diagnosis of 1 of the following hematologic conditions:

- Aplastic anemia with marrow aplasia, meeting all of the following criteria:

- Absolute neutrophil count < 500/mm^3

- Platelet and/or red cell transfusion dependent

- Chronic aplastic anemia, meeting all of the following criteria:

- Transfusion dependent

- Unresponsive to immunosuppressive therapy

- Alternative matched unrelated donor has been identified

- Congenital marrow failure syndrome, including any of the following (with closely
matched related or unrelated donor):

- Primary red cell aplasia (Diamond-Blackfan syndrome)

- Congenital neutropenia (Kostmann's syndrome)

- Amegakaryocytic thrombocytopenia

- Congenital dyserythropoietic anemias

- Other severe acquired cytopenias in which a transplantation using a
combined busulfan/cyclophosphamide conditioning regimen is indicated

- Hemoglobinopathy (with closely matched related or unrelated donor)

- β-thalassemia major

- Sickle cell anemia

- Hemoglobin E/β-thalassemia

- Severe immunodeficiency disease

- Chediak-Higashi disease

- Wiskott-Aldrich syndrome

- Combined immunodeficiency disease (Nezelof's)

- Hyper immunoglobulin M (IgM) syndrome

- Bare lymphocyte syndrome

- Chronic granulomatous disease

- Familial erythrohemophagocytic lymphohistiocytosis

- Other stem cell defects (e.g., osteopetrosis)

- Severe immune dysregulation/autoimmune disorders

- Achieved a transient response to prior immunosuppressive therapy

- Chronic myelogenous leukemia

- Disease in first chronic phase

- Acute myeloid leukemia

- Disease in first remission

- Myelodysplastic syndromes

- Inborn errors of metabolism

- Histiocytosis

- No severe combined immunodeficiency disease

- Matched related or unrelated donor available by high resolution DNA typing

- Related donor, meeting both of the following criteria:

- Matched at both human leukocyte antigen (HLA)-Drβ1 alleles

- No more than 1 mismatch at the 4 HLA-A and -B alleles

- Unrelated donor, meeting 1 of the following criteria:

- Marrow matched at both HLA-Drβ1 alleles AND no more than 1 mismatch at the
4 HLA-A and -B alleles

- Umbilical cord blood matched at 5/6 HLA-A, -B, and -DRβ1 alleles with at
least 1 -DRβ1 match AND there are ≥ 3x10^5 CD34+ (Cluster of
differentiation 34-positive) cells per kg body weight of recipient
available at the time of cryopreservation

PATIENT CHARACTERISTICS:

- Cardiac ejection fraction ≥ 27%

- Creatinine clearance ≥ 50 mL/min by 24-hour urine collection or glomerular filtration
rate

- DLCO (diffusion capacity of lung for carbon monoxide) ≥ 50% of predicted (corrected
for anemia/lung volume)

PRIOR CONCURRENT THERAPY:

- No prior transplantation for leukemia from which patient remains engrafted and
alemtuzumab is not needed as part of the conditioning regimen