The polymerase chain reaction (PCR) allows the detection of minimal amounts of nucleic sequences and has been successfully used to test for the chronic myeloid leukemia-specific bcr/abl transcripts. We studied blood samples from 17 patients who had undergone allogeneic bone marrow transplantation for CML, using a modified polymerase chain reaction-based assay for the detection of leukemic mRNA. This nested PCR technique was found to be highly sensitive, detecting the chimeric bcr/abl transcript in 16 of 17 patients including several long-term survivors. Cytogenetic techniques failed to detect Ph mitoses. The clinical significance of the persisting bcr/abl transcript for long periods following BMT is poorly understood and remains to be elucidated by further studies.