We have utilized the polymerase chain reaction (PCR) to sensitively detect the minimal residual leukemia in 24 patients who were hematologically normal and were negative for Ph1 chromosome after bone marrow transplantation (BMT). None of the patient showed breakpoint cluster region (BCR) gene rearrangement by Southern blot analysis, however, PCR technique revealed the bcr/abl mRNA in 13 of 24 patients. In patients who received CY + TBI as a conditioning regimen, 6 out of 8 patients were detected bcr/abl mRNA by PCR. On the other hand, patients who received high dose AraC+ CY + TBI or busulfan + CY as a conditioning, bcr/abl mRNA was detected in 4 out of 9 patients and 1 out of 5 patients, respectively. There was no clear association between the presence or absence of graft versus host disease and the detection of bcr/abl mRNA by PCR.