Detection of host cells in peripheral blood and/or bone marrow (mixed chimerism) of patients who have undergone bone marrow transplantation (BMT) is possible using either immunological methods or cytogenetic or molecular genetic analysis. We shall report a new method for the detection of mixed chimerism, which makes use of the fact, that the von Willebrand factor (vWF) gene has a highly variable region-variable number of tandem repeats (VNTR) — within intron 40. vWF-VNTR amplification by the polymerase chain reaction (PCR) was performed as described by Peake et al. [5]. We have studied 185 peripheral blood and/or bone marrow samples of 26 patients. Median time after BMT was 14 months (range 1–83 months). Of the 11 patients who were studied sequentially during the first 100 days following BMT, mixed chimerism was detected in four, but only transiently. None of these patients has relapsed so far. Of 18 patients who were studied more than 100 days after BMT mixed chimerism was found in three; two of these patients have subsequently relapsed. The advantages of this new method are: (a) it is informative in a high percentage of patients; (b) it requires only small amounts (200l) of peripheral blood; (c) reliable results can be obtained at leukocyte counts of even less than 50 perl. The clinical relevance and sensitivity of the method compared with established methods for detection of mixed chimerism remain to be determined.