Various lymphohaematopoietic compartments represented by cells from T-cell colonies, myeloid progenitor cells (CFU-GM), erythroid progenitor cells (BFU-E), and bone marrow after short-term culture (BM) have simultaneously been analysed in 15 patients receiving 17 bone marrow transplants for Philadelphia chromosome (Ph) positive chronic myeloid leukaemia (CML) or acute lymphoblastic leukaemia (ALL). The marrow grafts were not T-cell depleted. Ten patients without relapse did not show any myeloid cells of host origin until their last follow-up or until death. However, in four of these patients single lymphoid host cells not carrying the Ph chromosome were found after BMT without clinical consequences. In patients with cytogenetic or haematological relapse Ph positive metaphases were first detected in any of the progenitor cell compartments along with residual donor cells in two of three patients. BM became Ph positive after various time intervals. Another patient with CML became Ph positive in all compartments investigated at the same time. The only patient with Ph positive ALL remained completely Ph negative also when haematological and clinical relapse was evident. All patients with relapse exhibited complex clonal and non-clonal chromosomal aberrations at the time of recurrence of the Ph chromosome. Such abnormalities not identical to those usually found with evolution of the disease and preferentially occurring in progenitor cells preceded the reappearance of Ph positive metaphases in one of our patients.