We have reviewed literature data on 6,306 cases of hematological neoplasia--acute and chronic lymphatic and myeloid leukemias (CML excepted), myelodysplastic and chronic lymphoproliferative and myeloproliferative disorders, and malignant lymphomas--with the goal of quantitatively ascertaining how often cytogenetically unrelated clones occur in these diseases. Unexpectedly wide variations were found: in ANLL, unrelated clones were present in 1.1% of the 2,506 known cases with chromosome abnormalities characterized with banding technique; in the various myelodysplastic (MDS) and chronic myeloproliferative (CMD) disorders (total number of cases 1,299) the frequency was 4.3% and in lymphatic malignancies 1.3% (total case number 2,501). In the latter group the proportions varied between 0.4% and 0.6% in ALL and malignant lymphoma (ML) to as much as 6.2% in CLD and 7.3% in CLL. Some karyotypic abnormalities were encountered more often than would be expected from their general frequency in the various diseases. This discrepancy was particularly evident in MDS and CMD, where 5q- was found in slightly less and +8 in somewhat more than half of the 56 cases. Furthermore, these two aberrations were found as the only changes in the two coexisting clones in one-fourth of the material. Although if viewed in isolation these data would undoubtedly be best explained by assuming a multicellular origin of the neoplasm, it is entirely possible that what are cytogenetically perceived as unrelated clones could be subclones with some invisible aberration in common. If so, this interpretation indicates that changes like +8 and 5q-, both of which are common rearrangements in bone marrow neoplasms, are actually secondary changes that develop during tumor progression.