We have cytogenetically analyzed three primary adenocarcinomas of the lung. All tumors had chromosome numbers in the triploid region. The multiple structural aberrations included rearrangements of 3p, in two cases affecting the segment 3p14-23, where deletions are characteristically found in small cell lung carcinomas. Isochromosomes for 8q were present in two tumors and i(9q) in one tumor. In the few previously reported cytogenetic analyses of pulmonary adenocarcinomas, all of which examined metastases or cell lines, i(8q) was found in one case and i(9q) in two cases. These isochromosomes, therefore, represent previously unrecognized nonrandom changes in adenocarcinomas of the lung, and might constitute primary aberrations in this tumor type.