An 80 year-old woman presented subleukaemic acute monoblastic leukaemia (AML-M5a). Her bone marrow showed invasion by highly dysplastic histio-monocytic cells of great size and wide cytoplasm, with intense phagocytic activity (erythrophagocytosis was frequently seen), and with abnormal karyotype (50XX, +8, +8, +16, +21). The different malignant and reactive features of the mononuclear phagocytic system are commented, along with the haemophagocytic activity of the histio-monocytic cells in different states. The cytogenetic anomalies more frequently found in AML-M5 are also dealt with as compared to this patient's. The case reported here seems to correspond to subleukaemic acute "monophagocytic" leukaemia, with a biologic phenotype close to that of malignant histiocytosis.