We report derivative chromosomes, originally interpreted as 9q;15q whole-arm translocations, in tumor cells from two patients with non-small cell lung cancer (NSCLC). One of the tumors was diagnosed as an adenocarcinoma and the other as an adenosquamous carcinoma. In each case, there was no normal chromosome 9. Because of the pericentromeric location of the breakpoints, classical cytogenetic banding techniques did not permit determination of the centromeric origin of these derivative chromosomes. Fluorescence in situ hybridization (FISH) with satellite (alpha, beta, classical), ribosomal DNA, alpha-interferon (alpha-IFN), and whole chromosome painting probes indicated that the 9;15 rearrangement is dicentric in both tumors. In one of these cases, the derivative chromosome is interpreted as a dic(9;15) (p11;p11.2); the other case has a more complicated rearrangement involving reorientation of pericentromeric sequences. A 9q;15q whole-arm derivative chromosome was reported previously in another lung adenocarcinoma, suggesting that this abnormality may represent a recurrent change in lung carcinomas, particularly those displaying adenomatous features.