Fetomaternal microchimerism has been demonstrated, and immunologic tolerance to unshared HLA antigens between mother and offspring may be suggested. We used T-cell-repleted bone marrow transplantation (BMT) from their HLA-haploidentical mothers to treat 6 patients with fatal nonmalignant diseases. The number of mismatched HLA loci in the graft-versus-host disease (GVHD) direction was 3 in 4 patients and 2 in 2 patients. The number in the host-versus-graft direction was 3 in 4 patients, 2 in 1 patient, and 1 in 1 patient. Microchimerism of inherited paternal antigens was demonstrated in 5 donors, and microchimerism of noninherited maternal antigens was detected in 3 recipients. GVHD prophylaxis consisted of short-course methotrexate, tacrolimus, and mycophenolate mofetil (3 patients) or short-course methotrexate, tacrolimus, and methylprednisolone (1 patient). Engraftment was achieved in 5 patients who had received preconditioning, and T-cell engraftment was confirmed in 1 patient with severe combined immunodeficiency. Acute GVHD developed in 3 patients: grade 1 in 2 patients and grade 2 in 1 patient. Chronic GVHD was observed in 5 patients: localized type in 3 patients and extended type in 2 patients. Five patients were alive 11 to 30 months after BMT and 1 patient died of chronic GVHD. Unmanipulated haploidentical BMT from a maternal donor may be the treatment of choice of poor-prognosis nonmalignant diseases.