In chronic myelogenous leukemia (CML) malignant cells are characterised by the Philadelphia chromosome (Ph), resulting from a translocation t(9;22). The position of the breakpoint within the major breakpoint cluster region (M-bcr) on chromosome 22 has been shown to correlate with the clinical course of the disease or, more recently, thrombopoietic activity. We have therefore determined the breakpoint localisation in 53 Ph-positive CML patients. Following the 5'/3'-region definition of Inokuchi et al. Leukemia Research 15, 1067 (1991) [1], 22 of our patients have 5' and 31 of our patients have 3' orientated breaks. No correlation was found between platelet counts and breakpoint localisation.