Systemic mast cell disease (SMCD) can be regarded as a tumorous proliferation of tissue mast cells involving various organs. The frequency with which SMCD is found in patients with haematological disorders suggests that the association is non-random. The association includes primarily, myeloid disorders such as myelodysplastic syndromes and acute or chronic myeloproliferative disorders. Lymphoproliferative disorders may also occur but more rarely, mostly non-Hodgkin's low grade B cell lymphomas. In this report a case is described in which SMCD occurred in a patient with hairy cell leukaemia.