Fluorescent in situ hybridization (FISH) with biotinated chromosome-specific repetitive DNA probes was used for the cytogenetic study of lung tumors and three cell lines of human lung cancer. The authors utilized a set of satellite DNA probes, specific for chromosomes 7, 17, X, Y in order to detect numerical chromosome aberrations in tumor cell nuclei. Normal diploid human lymphocyte nuclei, which served as the control, have have two signal spots in 95% of nuclei in response to 7, 17 chromosome probes. However, lung cancer cells have numerical heterogeneity, and copy numbers as determined by FISH were not definite with each probe. Discrepancies between cytogenetic and flow cytometric studies in the detection of aneuploidy in some tumors were shown. The number of FISH spots showed a correlation only with the Ki-67 labeling index expressed in proliferating cells. Loss of the Y chromosome in a high percentage of cells was seen by FISH in some tumors from male patients. These data indicate that FISH with chromosome-specific repetitive DNA probes can serve as a cytogenetic tool for the analysis of interphase nuclei of lung tumors with respect to the detection of numerical chromosome abnormalities.