We identified two patients with acute leukemia in relapse (one lymphoblastic and the other with evidence of mixed lymphoid-myeloid differentiation) with t(12;17)(p12;q11) as the primary karyotypic abnormality. There are six previously reported cases of acute leukemia with an identical or similar translocation. To our knowledge, t(12;17) has not been reported in other forms of neoplasia. A review of these cases suggests that t(12;17) carries a poor prognosis.