Osteocalcin (OCN) is defined as a marker for osteoblasts but incompletely spliced variants as characterised by intron retention may also occur in non-osseous tissues including tumour cells. Bone marrow of haematological diseases was shown to harbour a stem cell candidate common to MSC (mesenchymal stem cells) and HSC (haematopoietic stem cells), which expressed OCN. In this study, we analysed cells from bone marrow samples obtained from 4 patients with AMLs, one patient with CML at blast crisis, one CML patient in chronic phase and 7 patients with myeloproliferative disease (MPD), as well as stem cells from peripheral blood and bone marrow obtained from healthy donors. In attempt to find out whether the presence of such stem cells is associated with disease status, sequential bone marrow samples from a patient with acute myeloid leukaemia (AML/M2Eo) were analysed at 6 time points including diagnosis, remission (= months 4, 5 and 17 after diagnosis) and relapse (months 29 and 41 after diagnosis). RT-PCR analyses were performed to characterise spliced and unspliced mRNA variants of OCN and to quantify mRNA levels of c-KIT and the Runt-transcription factors AML1 and AML3, known to regulate gene expression of c-KIT and OCN. Results of immunostaining for c-KIT and OCN showed that positive signals for OCN were detected only in those bone marrow smears, which expressed spliced OCN-mRNA, i.e. the bone marrow samples from the AML/M2Eo-patient in the relapse phase, the 2 samples of AML/M2, the sample of CML at blast crisis. The same samples and all others, which were positive for the unspliced OCN-mRNA, were also positive for c-KIT (both at the protein and at the mRNA level) as well as for AML1- and AML3-mRNA, however, at lower quantities. Our data indicate an association of OCNs expression with haematological malignancies at disease stages with activated bone marrow stem cells.