The classification of the acute leukemias depends mainly on the morphologic and cytochemical evaluation of the blast forms. One of the main accepted morphologic criteria in the differentiation between acute lymphoblastic leukemia (ALL) and acute myeloblastic leukemia (AML) is the absence of granules in the blast cells of ALL. We evaluated a patient with ALL in whom granules were present in the cytoplasm of 35% of the blast cells, as seen in AML. Cytochemical evaluation was performed, including periodic acid-Schiff reaction, Sudan black B, alpha-naphthyl acetate, alpha-naphthyl butyrate, naphthol AS-D chloroacetate, and acid phosphatase stains. The results of these studies confirmed the morphologic impression and diagnosis of ALL. Ultrastructural evaluation revealed that the granules consisted of many tiny vesicles closely packed together in a proteinaceous matrix, resembling to some extent the inclusions described in lymphocytes in the Chédiak-Higashi syndrome, but clearly different. The morphologic, cytochemical, and ultrastructural studies of this unique case are presented in detail. To our knowledge, this is the first time that such granules have been described in blast cells of ALL.