We present the case of a 60-year-old woman with drug-induced aplastic anaemia with a healthy monozygotic twin. Proof of monozygosity was confirmed by studies using the hypervariable minisatellite probe to obtain identical DNA fingerprints in donor and recipient. In vitro co-culture studies performed showed no evidence of a recipient-derived cellular or humoral inhibitor of donor haemopoiesis. Despite this, there was no engraftment following simple marrow infusion without preconditioning. A second syngeneic transplant following high dose cyclophosphamide therapy produced trilineage engraftment but severe thrombocytopenia developed at 3 months, followed later by pancytopenia with generalized marrow failure. Following a third syngeneic transplant with cyclophosphamide and total lymphoid irradiation there was good initial engraftment but graft failure occurred at 14 weeks. A fourth transplant using Campath 1G as preconditioning resulted in no engraftment and the patient died of septicaemia 8 weeks following her fourth transplant. We suggest that the cause of the recurrent aplastic anaemia in this case was a defect of marrow stroma as neither an inhibitor of donor haemopoiesis nor an intrinsic defect of donor stem cell growth could be demonstrated in vitro.