Juvenile chronic myelomonocytic leukemia (JCMMoL) is a rare disease with no specific type of chromosome aberration yet delineated. We report a 2-year-old boy who had in his leukemic bone marrow (BM) and peripheral blood (PB) cells the 46,XY,der(15)t(3;15)(q13.1;q26) karyotype. Phytohemagglutinin (PHA)-stimulated lymphocytes of peripheral blood had a normal 46,XY karyotype. The origin of the duplicated part of 3q was proved by fluorescence in situ hybridization (FISH) with the pHSR(sat III 15p) DNA probe and a chromosome 3-specific DNA library (i.e., chromosome painting). The chromosome finding in our case provides further proof of the close relationship between the rearrangement in region 3q13-->3q26 and the pathogenesis of acute myeloid leukemia (AML). Our patient has transformed into erythroleukemia [M6 according to the French-American-British (FAB) classification] during the course of the disease.