Six young patients with X-linked agammaglobulinemia and proven mutations in Btk were treated with cord blood or bone marrow transplants from HLA-matched siblings. Complete blood counts, serum chemistries, serum immunoglobulin concentrations, lymphocyte cell surface markers, and physical findings were evaluated at 3- to 5-day intervals for the first 2 weeks after transplant and then every 3 to 6 months. The first three patients were not given any preparative regimen or antirejection drugs and at 24 to 42 months posttransplant these patients have shown no benefit or harm related to the transplants. The second three patients were not given a preparative regimen but were treated with cyclosporine A (70 days) and mycophenolate mophetil (28 days) after transplant. Two of these patients have developed normal sized, nontender cervical lymph nodes 3 to 12 months after transplant but none of the three patients have shown an increase in serum IgM or an increase in the number of peripheral blood B cells. It is likely that successful engraftment will require more aggressive immunosupressive medications.