We studied the clinical, morphological, and immunologic characteristics of 11 patients with 11q translocation-associated acute leukemia. There were three patients with t(9;11)(p22;q23), one with a variant of the t(9;11), three with t(11;19)(q23;p13), two with t(1;11)(p32;q23), one with t(10;11)(p15;q22or23), and one with t(11;17) (q23;q25). The breakpoints in chromosome 11 clustered in band q23. The morphological feature was FAB-M5 in two patients, FAB-M2 in one, FAB-L1 in six, and lymphoblastic lymphoma in one. The remaining patient underwent morphological changes from FAB-L1 seen at the time of diagnosis to M5b at relapse. Immunologic marker studies in ten patients revealed that one had T cell type; another pre-B cell type; three CALLA- Ia- non-T, non-B type; two CAL-LA- Ia+ non-T, non-B type; two monocytic type (positive Fc-receptor); and the remaining one underwent phenotypic changes from CALLA+ Ia+ non-T, non-B type to monocytic type. The patients were usually young; five were under 1 year and two were 9 and 13 years. Hyperleukocytosis was observed in eight of the ten patients with acute leukemia, and two of the eight died of intracranial hemorrhage within two days of admission, associated with disseminated intravascular coagulation. These findings indicate that leukemia with the 11q23 translocation share certain characteristics in common, irrespective of the recipient chromosome, even though the latter may have some influence on the morphological and immunologic phenotype. Our data provide a hypothesis that multipotent stem cells are involved in the genesis of the 11q translocation-associated leukemia.