Two patients with acute promyelocytic leukemia had an identical chromosomal abnormality detected by fluoresecence banding. In each case, the clinical course was rapidly fatal, and was characterized by a lack of response to chemotherapy with cytarabine and thioguanine, and was complicated by disseminated intravascular coagulation. Bone marrow cells from each patient contained 46 chromosomes; in each instance, however, one chromosome 17 had a deletion of almost one half of the proximal portion of the long arm [del(17)(q11q21 or 22)].