We investigated 23 patients for their chimerism status who underwent allogeneic transplantation using peripheral blood progenitor cells (PBPCT) for chronic myelogenous leukemia (CML) (n = 14), acute myelogenous leukemia (AML) (n = 5), acute lymphoblastic leukemia (n = 1), myelodysplasia (MDS) (n = 1), and Hodgkin's disease (HD) (n = 2). These data were compared with those of patients after allogeneic BMT after matching them for disease and disease stage, sex of donor and recipient, GVHD prophylaxis, conditioning therapy and degree of HLA disparity. Patients were studied monthly up to 16 months post-transplant. In 11 of 23 (48%) patients who were transplanted with PBPCs and in 18 of 23 (78%) patients after BMT a mixed chimerism was detected at 1 month post-transplant. After 3 months, six of 21 (29%) evaluable patients after PBPCT remained mixed chimeric as opposed to 12 of 21 (57%) patients after BMT. We also assessed minimal residual disease using detection of the chimeric BCR/ABL transcripts by PCR of CML patients in this study. In four of 14 (29%) patients who underwent PBPCT, the BCR/ABL chimeric transcript was detected, while after BMT eight of 14 (57%) CML patients remained BCR/ABL positive. In two of these BMT patients, a cytogenetic relapse developed subsequently, and one other patient suffered a hematological relapse, whereas one of the CML patients relapsed after PBPCT. The present data may indicate that after PBPCT the incidence of leukemic relapse is similar or even lower than after BMT.