A systematic cytogenetic study was performed in 49 patients with polycythemia vera (PV) in order to investigate the occurrence of subtelomeric rearrangements of chromosome 20, the most frequently rearranged chromosome in this myeloproliferative disorder. Partial deletion of the long arm of chromosome 20 was observed in two patients and two cryptic translocations, t(1;20)(p36;q13) and t(18;20)(p11;q13) in two others, all previously treated. The localization of the breakpoints of the translocated 20 chromosomes was different in the two translocations, as shown by fluorescence in situ hybridization (FISH) to metaphase chromosomes using BAC clones. Although infrequent (2/49), cryptic translocations of chromosome 20 deserve to be detected as preliminary to identification of molecular defects in PV.