Chimerism studies employing PCR and Southern techniques targeting VNTR loci were performed in 17 severe acquired aplastic anemia patients who were long-term survivors after BMT. They were studied a median of 4 years after BMT (1-12). All patients had normal blood counts. All patients conditioned with radiation-based schemes showed a full donor pattern of hemopoiesis. Conversely, out of five patients who received only cyclophosphamide as conditioning therapy, two of them had a late graft failure (2.4 and 3 years after BMT). One of these relapsing patients had a durable mixed chimerism, which was first detected 1 month after BMT. Our results seem to suggest that durable mixed chimerism can antecede graft failure in some patients conditioned only with cyclophosphamide, and that a more stringent monitoring can be clinically rewarding in this group of patients.