Diagnostic relevance of clonal cytogenetic aberrations in malignant soft-tissue tumors. BACKGROUND. Malignant soft-tissue tumors often present substantial diagnostic challenges. Chromosome aberrations that might be diagnostic have been identified in some types of soft-tissue tumors, but the overall frequency and diagnostic relevance of these aberrations have not been established. METHODS. We attempted to determine the karyotypes of a series of 62 consecutive, unselected malignant spindle-cell or small round-cell soft-tissue tumors (from 46 adults and 16 children) after direct harvesting of cells or short-term culture. All tumors were examined independently by immunohistochemical staining in addition to routine light-microscopical evaluation, and all but two tumors were examined by electron microscopy. RESULTS. Metaphases were obtained from 61 of the 62 tumors, and clonal chromosome aberrations were identified in 55 (89 percent). In the six tumors that yielded metaphases but lacked apparent clonal aberrations, the normal metaphases were found to originate from non-neoplastic stromal elements within the tumor specimens. Thus, all tumors in which karyotyping was successful contained clonal chromosome aberrations. Forty of 62 tumors (65 percent) contained clonal chromosome aberrations that either suggested or confirmed a specific diagnosis; in 15 of these tumors (24 percent of all tumors), the aberrations were important in establishing the final diagnosis. Cytogenetic analyses were particularly informative about small round-cell tumors from children: 8 of 14 round-cell tumors contained diagnostically important chromosome aberrations. Using the combined approaches of light and electron microscopy, immunohistochemistry, and cytogenetics, we established an unambiguous diagnosis for 60 of 62 tumors. CONCLUSIONS. Cytogenetic analyses reveal clonal chromosome aberrations in virtually all malignant soft-tissue tumors. These clonal chromosome aberrations, particularly in small round-cell tumors in children, often have diagnostic relevance. 