In a thalassaemic female carrying a 46, XX/46, XXqi mosaic, the occurrence of acute myeloblastic leukaemia was observed. Karyotype analysis of several bone marrow aspirates disclosed, besides both normal metaphases and metaphases containing the isochromosome, a line characterised by a supernumerary chromosome in the C group (C trisomy). A clonal proliferation of the leukaemic line from normal precursors was suggested since no cells carrying the isochromosome and the C trisomy together were found. In the course of the disease a remarkable plasma-cellular proliferation brought about a hyperglobulinaemia of monoclonal aspect (IgG, λ-type). Since the chromosomal analysis demonstrated that the presence of a C trisomy might also be present in the plasma cells, a close proliferative connection has been put forward for the stem cells differentiating along both the granulocytic and the plasmacytic line.