We report two cases of myeloproliferative syndromes in which the only karyotypic abnormality was an isochromosome of the long arm of chromosome 17. Because i(17q) is a nonrandom structural aberration found in nearly 12% of cases of Philadelphia (Ph)-positive chronic myelogenous leukemia (CML), we carried out a molecular analysis of the breakpoint cluster region (bcr) to verify the presence of genomic rearrangements characteristic of CML. The interest of the study was strengthened by the fact that i(17q) is frequently seen in CML and by recent reports showing that genomic changes of c-abl and bcr genes can be present even in the absence of a Ph chromosome. One of the two patients showed the presence of a rearranged fragment within the bcr, suggesting a Ph-positive CML diagnosis.