Chromosome abnormalities have been reported in more than 46,000 benign and malignant neoplastic disorders, leading to the identification of numerous recurrent abnormalities. A substantial number of recurrent balanced aberrations (RBAs), in particular, reciprocal translocations, occur with remarkable specificity in association with clinical and tumor characteristics. This information has become increasingly important both in basic cancer research, as a means to identify pathogenetically important genes, and clinically, as a diagnostic and prognostic instrument. Knowledge of the frequencies of such aberrations thus is of theoretical as well as practical value. However, it is unknown to what extent the data available in the literature reflect reality. A large proportion of the published cases, at least 40%, are biased, in the sense that they were reported because of a specific or unusual karyotypic feature. We have systematically ascertained all RBAs and present data on the frequencies of these abnormalities and their molecular genetic consequences among unselected patients, that is, those studied as part of investigations of consecutive series of individuals with a particular neoplastic disorder. The salient features of the present study are: (1) published data clearly overestimate the prevalence of individual RBAs in most tumor types as well as the proportion of patients having such aberrations. In fact, several well-known published RBAs are not recurrent or have not even been seen among unselected patients, and in no tumor entity, except for chronic myeloid leukemia, does the frequency of unselected cytogenetically abnormal neoplasms with RBAs exceed 35%; (2) the proportions of unselected cases characterized by RBAs among those tumor entities in which at least one RBA has been identified vary considerably both within and among hematologic malignancies, malignant lymphomas, and solid tumors; and (3) the molecular consequences of a substantial proportion, ranging from 19% in hematologic malignancies to 65% in epithelial tumors, of the most common RBAs in unselected patients remain to be clarified.