A new reciprocal, apparently balanced translocation between chromosomes 12 and 19, t(12;19)(q13;q13.3), was detected in 5% (3/59) of patients with FAB M1 or M2 acute nonlymphocytic leukemia. In either case, this translocation was part of complex but different cytogenetic abnormalities. None of the patients had a significant response to therapy. In one instance, however, the translocation was found at first relapse after 2 years of complete remission, and no information regarding the karyotype at disease onset was available. Hematologically common to these patients were marked marrow erythroid hyperplasia and severely abnormal erythropoiesis despite normal serum B12 and folate levels. A direct association between t(12;19) and these hematologic findings will have to be established as more cases with this chromosomal abnormality are identified.