In order to ascertain the frequency and distribution of isochromosomes in neoplasia, we surveyed the cytogenetic data from 20,007 tumors with clonal chromosome aberrations reported in the literature. Tumor types for which at least 50 cases with acquired aberrations and 10 cases with isochromosomes had been reported were selected, yielding a total of 18,160 neoplasms. Of these, 1,792 cases (9.9%) displayed a total of 2,014 isochromosomes. The 9 most common isochromosomes (detected in at least 50 cases) were, in decreasing order of frequency, i(17q), i(8q), i(1q), i(12p), i(6p), i(7q), i(9q), i(5p), and i(21q). The frequency of isochromosomes varied among the different tumor types, with the highest incidence in germ cell neoplasms (60%) and the lowest in chronic myeloproliferative disorders (2.3%). Also, the spectrum of isochromosomes differed among the neoplasms. The most common isochromosomes in the different tumor types were i(11q), i(17q), and i(21q) in acute myeloid leukemia; i(9q), i(17q), and i(22q) in chronic myeloid leukemia; i(17q) in chronic myeloproliferative disorders; i(X)(q13), i(17q), and i(21q) in myelodysplastic syndromes; i(7q), i(9q), and i(17q) in acute lymphoblastic leukemia; i(1q), i(7q), i(8q), and i(17q) in chronic lymphoproliferative disorders; i(1q), i(6p), i(9p), i(17q), and i(21q) in Hodgkin's disease; i(1q), i(6p), and i(17q) in non-Hodgkin's lymphoma; i(1q), i(8q), and i(17q) in adenocarcinoma; i(1q), i(3q), i(5p), and i(8q) in squamous cell carcinoma; i(5p), i(8q), and i(11q) in transitional cell carcinoma; i(1q), i(7q), and i(17q) in Wilms' tumor; i(1q), i(12p), and i(17q) in germ cell neoplasms; i(1p), i(1q), i(6p), and i(17q) in sarcoma; i(5p), i(6p), i(7p), and i(21q) in mesothelioma; i(1q), i(6p), and i(17q) in malignant neurogenic neoplasms; i(1q), i(6p), and i(17q) in retinoblastoma; and i(1q), i(6p), and i(8q) in malignant melanoma.