A modification of the polymerase chain reaction (PCR) has been successfully used to detect minimal residual leukemic cells in chronic myelogenous leukemia (CML) patients after bone marrow transplantation (BMT), by amplification of the transcript of the specific bcr/abl mRNA. The capability of detecting CML cells was 10(-6) by our procedure. We studied peripheral blood and bone marrow samples from 8 CML patients observed up to 62 months after BMT. In two patients who were studied for the first time at 62 and 59 months after BMT, bcr/ab mRNA could not be detected in all samples. In 4 patients, who were studied within 1.5 years, bcr/abl mRNA was detected in all serial samples. In the other two patients, bcr/abl mRNA was detected in their peripheral blood samples, but not in their bone marrow samples which were obtained simultaneously. The reason why bcr/abl mRNA was not detected in bone marrow samples remains unknown. There were no differences in the pattern of mRNA chimerism, that is, b2aII (2 patients) or b3aII (4 patients), both before and after BMT. Cytogenetic relapse was documented at 5 months after BMT in one patient but all the other patients have remained in remission. Our results suggest that CML cells persist throughout the conditioning regimen for a considerable period of time and eradication of CML cells may occur several years after BMT.