Translocation (Y;1)(q12;q12) is a rare cytogenetic anomaly occurring in hematologic disorders thought to affect stem cells. We report here on two new cases, one end-stage myelofibrosis and one chronic myelomonocytic leukemia. The translocation breakpoints were assessed by conventional cytogenetic techniques in both cases and by FISH in the second case. A partial trisomy of the 1q21-qter region could be demonstrated. The data of the literature are reviewed and the possible pathogenetic mechanisms are discussed.