Inclusion Criteria:

- Life-threatening marrow failure of nonmalignant etiology meeting two of the three
following criteria: granulocytes < 500/mm^3; a corrected reticulocyte count < 1%;
platelet count < 20,000/mm^3

- Failure to respond to the best available immunosuppressive treatment protocol by 75
days after initiation of therapy

- Lack of an HLA-identical family member or closely matched (9 or 10 of 10 HLA-locus
match) unrelated marrow donor

- DONOR: Unrelated UCB unit matched for at least 4 of 6 loci

- DONOR: Related UCB unit matched for at least 3 of 6 lock

- Selection of the UCB unit(s) will be based upon matching or mismatching at HLA-A, B
antigen level and DRB1 allele level typing; while HLA-C antigen/allele and HLA-DQB1
antigen/allele level typing are not considered in the matching criteria, if available
each may be used to optimize unit selection

- Multiple UCB units are allowed to provide sufficient cell dose; when multiple units
are selected, the following rules apply: a) the UCB unit with the least HLA disparity
will be selected first (i.e., selection priority is 6/6 match > 5/6 match > 4/6
match), additional UCB units may be selected to increase cell dose; b) UCB units must
be matched to each other for at least 4 of 6 loci; c) each unit must contain at least
1.5 x 10^7 Total Nucleated Cells per kg recipient weight; d) the total cell dose of
the combined units must be at least 3.0 x 10^7 Total Nucleated Cells per kg recipient
weight

Exclusion Criteria:

- Severe disease other than aplastic anemia that would severely limit the probability
of survival during the graft procedure; patients who present with active fungal
infections must be treated to resolve this problem before beginning the conditioning
regimen

- HIV seropositive patients

- Patients who have developed clonal cytogenetic abnormalities or a myelodysplastic
syndrome (these patients will be considered in separate protocols for myelodysplastic
syndrome, etc.)

- Patients with paroxysmal nocturnal hemoglobinuria or Fanconi anemia

- Patients > 40 years of age

- Related or unrelated cord blood units with < 1.5 x 10^7 Total Nucleated Cells per kg
recipient weight

- Related or unrelated cord blood units without full testing and negative results for
hepatitis A, B, C, HIV, HTLV-1, CMV viruses