In view of the sparsity of reports on nucleolar organizer regions (NORs) in human tumor metaphase chromosomes, we have applied the silver (Ag-NOR) technique to a previously studied testicular germ-cell tumor that had an abnormal translocation, which involved a 13p, and to nine new sequentially studied tumors. Six of the new tumors, and the germ cell tumor, showed ectopic NORs (e.g., at the end of the long arm of acrocentrics or metacentrics, or interstitially in metacentrics): five carcinomas and a leiomyosarcoma, all of which also revealed numerous structural chromosome changes after G-banding. The three tumors that did not show ectopic NORs were lymphomas with relatively simple karyotypic changes. It seems that the presence of ectopic NORs in the majority of the tumors is a reflection of the multiplicity of structural changes in these tumors and does not signify that there is any particular propensity for acrocentrics to take part in these changes. It was interesting that several of the chromosomes showed large notably a metacentric in a squamous cell carcinoma of the skin in which the Ag-NOR-positive region was seen as an unstained gap in unbanded and G-banded chromosomes.