In peripheral blood of at least 50% of healthy individuals, the translocations t(9;22) BCR/ABL, t(14;18) IgH/BCL-2, t(2;5) NPM-ALK and MLL duplications, which characterize chronic myelogenous leukemia and acute lymphoblastic leukemia, follicular lymphoma, anaplastic large cell lymphoma, and acute myelogenous leukemia, respectively, are detectable by sensitive polymerase chain reaction (PCR). No structural differences between these aberrations in normal or disturbed hematopoiesis are apparent. While the total count of t(9;22)- and t(14;18)-positive cells does not exceed 10(4), those with MLL duplications are more frequent and account for approximately 10(7) cells in the total blood pool. t(14;18)-positive cells seem to be immortalized, but the biological consequences of the other aberrations in positive healthy persons have not been studied in detail. Due to the high frequency of positive individuals, most of them will not suffer from the correspondent leukemia or lymphoma, and criteria for subgroups that may be at a higher risk remain to be determined. Most likely, the number of genetic aberrations in healthy individuals, which so far are only associated with hematopoietic disorders, will increase in the near future.