Minimally differentiated acute myeloid leukaemia (AML-M0) may pose difficulty in diagnosis since morphological criteria alone are not reliable. Other studies such as electron microscopy, immunocytochemistry and surface marker analysis are needed. The role of cytochemistry has been limited to the negative staining of blasts for Sudan Black B and myeloperoxidase. An abnormal morphology and cytochemistry of bone marrow maturing cells may indicate the myeloid nature of acute leukaemia. In this case of AML-M0, increased numbers of maturing and mature bone marrow granulocytic cells staining simultaneously for both specific and non-specific esterase (double esterase) are described. In acute leukaemia, this abnormal cytochemical finding seems to be specific for myeloid leukaemia and may be used as supplementary evidence of myeloid differentiation of morphologically undifferentiated blasts in cases of AML-M0.