We studied the role of wt-1 as a minimal residual disease (MRD) marker in 46 patients with acute leukemia (AL) (1st CR n = 24; 2nd CR n = 9, in relapse n = 13) after allogeneic bone marrow or peripheral blood stem cell transplantation. Prior to allogeneic transplant, wt-1 transcripts were detected by PCR in 38 of 46 patients (83%) with AL. After transplant, in 14 of 38 patients (37%) wt-1 transcripts were detected in at least one PCR assay at a median of 12 months post transplant (range 1-89 months). Twelve of the 38 patients relapsed after transplant, but only seven of the 12 were wt-1 positive after transplant. In five relapsing patients the wt-1 test remained negative 0 to 3 months prior to relapse. On the other hand, only seven of 14 patients with a positive test for wt-1 after transplant, relapsed consecutively. In 17 of the 46 study patients chromosomal abnormalities had been found prior to transplant (AML-M4eo with inv16 n = 7, AML-M2 with t(8;21) n = 3, AML-M3 with t(15;17) n = 1, AML-M5 with t(4;11) n = 1, ALL with t(9;22) n = 5). In these 17 patients, we analyzed the wt-1 transcript simultaneously with a specific chimeric transcript characteristic for the corresponding chromosomal abnormality. In 32 of 45 samples (71%) the results for the MRD marker and wt-1 transcript were concordant, but differed in 13 patients. We conclude that detection of wt-1 transcripts does not predict leukemic relapse reliably and is therefore not a suitable MRD marker in patients with acute leukemia after allogeneic BM or PBSC transplantation. Bone Marrow Transplantation (2000) 25, 91-96.