The lymphoproliferative disorders and the myelodysplastic syndromes are thought to result from the clonal expansion of a single abnormal precursor cell. Seven patients who were found to have concurrent myelodysplasia and lymphoproliferation are described. In all cases myelodysplasia and lymphoproliferation were diagnosed simultaneously or myelodysplasia preceded overt lymphoproliferation by a brief interval. Thus, myelodysplasia in these patients was not related to treatment given for a lymphoproliferative disorder. Stem cell theory is reviewed with special reference to evidence for an adult human stem cell common to both haemopoietic and lymphoid cell lines (CFU-L-M). The presence of concurrent myelodysplasia and lymphoproliferation in these seven patients may be the result of the clonal expansion of an abnormal pluripotential stem cell. It may be possible in some cases to achieve remission of the lymphoproliferative disorder with return of normal haematopoiesis if residual normal stem cells are present and repopulate the marrow following cytotoxic therapy.