Acute myeloblastic leukemia occurs as a complication of myelodysplastic syndromes, but the appearance of an acute lymphoblastic leukemia (ALL) has only been reported once. We describe a case in which lymphoblastic transformation occurred in the setting of a dysmyelopoietic syndrome. This leukemia was characterized by lymphoid morphology, terminal deoxynucleotidyl transferase (TdT) positivity, cytogenetic abnormalities, and immunoglobulin gene rearrangements. The patient responded to conventional therapy for this leukemia (vincristine and prednisone). Our case supports the hypothesis of a common lymphohemopoietic progenitor and suggests that in vitro tests may help identify a subset of these patients and be important in selecting appropriate therapy.