Fragile site studies were performed on a total of 126 patients with leukemia and other hematologic disorders including myelodysplastic syndrome (MDS) and polycythemia vera (PV). Compared with an incidence (6.0%) of heritable rare fragile sites in the healthy population, the frequency was not higher in the patient group (3.2%), as a whole. However, two cases of fra(17)(p12) in MDS appeared fourfold larger than expected for this group of patients. In one case, a homozygous carrier of fra(17)(p12) in PV was also very rarely expected from its population incidence. These findings suggested a possible role of rare fragile sites, at least in the etiology of these preleukemic or myeloproliferative disorders.