Atypical promyelocytic leukemia (M3) with immature primary granules and t(15;17)

An unusual case of acute myeloid leukemia with a standard t(15;17) is described. While light microscopy morphology was suggestive of acute myeloid leukemia M5a and light microscopy cytochemistry showed 80% of blasts to be strongly positive with Sudan Black B--more consistent with a diagnosis of M4--ultrastructural analysis demonstrated that the predominant cells were promyelocytes with immature primary granules hardly visible with the Romanovsky stains by light microscopy. Because typical cytologic and clinical features of M3 or M3 variant were lacking this atypical case would not have been recognized but for the presence of t(15;17) and the demonstration of promyelocytic features by electron microscopy.