An adult patient with Sudan Black B (SB B) positive leukaemic lymphoblasts is described. Peroxidase and naphtol AS D chloroacetate esterase stains were negative. The diagnosis of acute lymphoblastic leukaemia (ALL) was based on morphology (FAB classification: L1), on immunological marker studies (cALL+, Tdt+, Ia+) and on electron microscopy, revealed blasts, compatible with lymphoblasts. Additional proof of the diagnosis of ALL were the diffuse lymphadenopathy and the rapid response to ALL chemotherapy. Scattered azurophilic granules were present in some lymphoblasts; ultrastructurally multiple lysosomal inclusions were detected, containing small vesicles, sometimes in association with entrapped cytoplasmic organelles. The large amounts of phospholipids in these inclusions explain the Sudan Black B positivity. This type of ALL can easily be misdiagnosed as acute myeloid leukaemia.