A 57-year-old female patient, admitted for an acute abdominal syndrome, was found to have an extensive proliferation of mast cells both in the peripheral blood and the bone marrow. Cytochemical studies confirmed the mast cell characteristics of the pathological cell population, while the immunophenotype strongly suggested a bone marrow origin of this malignancy. The course of the disease was not affected by antiproliferative treatment and the patient, after progressive general deterioration, died of intractable haemorrhage. On both clinical and haematological criteria it seems possible to distinguish this rare case of primary mast leukaemia from the more common form of tissue mastocytosis with secondary leukaemia.