PROTOCOL OUTLINE: This is a randomized, multicenter study. Patients are randomized to receive cyclophosphamide IV over 60 minutes on days -5 to -2 with or without antithymocyte globulin IV over 4 hours.

All patients then receive bone marrow over 60-120 minutes on day 0, 36 hours after the last dose of cyclophosphamide.

Patients are followed at day 100, at 6 months, and at 1 year posttransplant.

PROTOCOL ENTRY CRITERIA:

--Disease Characteristics--

Severe aplastic anemia with the following criteria:

Hypocellular bone marrow with cellularity less than 20%

At least 2 of the following hematologic abnormalities:

• Neutrophil count no greater than 500/mm3
• Platelet count no greater than 20,000/mm3
• Reticulocyte count no greater than 50,000/mm3

HLA-identical sibling donor available

No clonal cytogenetic abnormalities, paroxysmal nocturnal hemoglobinuria, or myelodysplastic syndrome within 3 months of diagnosis of aplastic anemia

No congenital or constitutional aplastic anemia or Fanconi anemia

--Patient Characteristics--

Hepatic: Bilirubin less than 3 times upper limit of normal (ULN)

Renal: Creatinine less than 2 times ULN

Cardiovascular: Normal cardiac function

Other:

• No uncontrolled infection
• No severe concurrent disease
• HIV negative
• Fertile patients must use effective contraception