Cancer and Leukemia Group B demonstrated that adults with acute lymphoid leukemia (ALL) possessing blast cells with myeloid antigens (My+ALL), as identified by monoclonal antibodies against CD13 and CD33, have a worse prognosis than those lacking myeloid antigens (My-ALL). Consequently, we further studied this group of adults with ALL to determine if these immunological groups could be distinguished by morphological and cytochemical criteria. Bone marrow films were classified according to French-American-British Co-operative Group Criteria, assessed for myelodysplasia, and examined for blasts with azurophilic granules. More cases of My+ALL had L2 morphology than did My-ALL (68% vs. 49%, p = 0.04), and more cases of My+ALL were positive for acid alpha-naphthyl acetate esterase (61% vs. 31%, p = 0.03). The presence of myelodysplastic changes was not significantly different in My+ALL (13%) as compared to My-ALL (5%), but more cases of My+ALL had unusual blasts (monocytoid features and cytoplasmic buds) than did My-ALL (19% vs. 0%, p less than 0.01). In addition, more cases of My+ALL had greater than 5% of the blasts with azurophilic granules (42% vs. 13%, p = 0.01). In the My+ALL group the presence of azurophilic granules was associated with a longer median survival (13.5 months vs. 1.5 months, p less than 0.01). We conclude that My+ALL can be suspected when cases possess L2 morphology, unusual blasts, positive staining for acid alpha-naphthyl acetate esterase, and greater than 5% azurophilic granules. In addition, the poor risk group (My+ALL) can be further subdivided into better and poorer risk subgroups based on the presence of azurophilic granules.