Five patients received bone marrow allografts for leukaemia from serologically fully matched unrelated donors. The conditioning regimen was busulphan 16 mg/kg and cyclophosphamide (CY) 120 mg/kg. Engraftment was achieved in only two patients, with primary graft failure occurring in three patients. Two of the three patients with no evidence of myeloid engraftment on day 28 received GM-CSF without response. Both these patients required rescue with cryopreserved autologous material. Three patients remain alive, one died of relapse after a successful allograft and one died with pancytopenia. Although the number of patients is small, we conclude that the BuCY2 regimen with 120 mg/kg CY, while adequate for sibling allografts, may not be sufficiently immunosuppressive to permit consistent engraftment of bone marrow from unrelated donors.