Haemopoietic reconstitution was evaluated in 44 patients given HLA compatible sibling bone marrow transplants. The mean peripheral blood haemoglobin, neutrophil and platelet counts were markedly reduced early post-graft but returned to normal by 26 weeks post transplant. Bone marrow multipotent (CFU-Mix), erythroid (BFU-E) and myeloid (CFU-GM) progenitor cell reconstitution were also assessed at regular intervals up to 2 years post-graft. The mean value of CFU-GM increased gradually and attained a normal value by 52 weeks. The BFU-E value did not reach a normal value until after 52 weeks post-graft. However, CFU-Mix growth appeared to be impaired even up to 2 years post-transplant. The occurrence of graft versus host disease at 3 months post-transplant was associated with significantly lower mean numbers of platelets, marrow CFU-GM, BFU-E and CFU-Mix. Post-transplant patients who were on methotrexate therapy were also shown to have lower marrow CFU-GM and neutrophil values compared to those patients who received cyclosporin post-transplant. This study demonstrated that although peripheral blood counts were normal after 26 weeks post-graft, marrow stem cell reserve in these patients was reduced. This might in part explain the documented increase in risk of severe infections or thrombocytopenia in some of these patients, particularly during viral infection, graft-versus-host disease or immunosuppressive treatment.