A unique form of pulmonary malignancy develops in cockatiels. This report describes the gross, histologic, electron microscopic, and immunohistochemical features of this tumor in 6 cockatiels. DNA in-situ hybridization for polyomavirus in the neoplasm was also performed. The tumor was comprised predominantly of compact sheets of anaplastic round to polygonal cells. All tumors had a high mitotic index, and had occasional large clear to slightly basophilic intranuclear inclusions that caused peripheral dispersal or complete masking of chromatin. Tumors were invasive but convincing metastases were not observed. Transmission electron microscopy identified intracytoplasmic intermediate filaments, desmosomes between cells, and intranuclear cytoplasmic invaginations corresponding to the intranuclear inclusions in light microscopic sections. Neoplastic cells stained positive for vimentin, lysozyme, and in 1 bird, pan cytokeratin. All 6 pulmonary neoplasms were negative for avian polyomavirus using the FN-19/FN-40 cocktail and the long VP-1 probe. We propose that these tumors may be poorly differentiated carcinomas of pulmonary or thymic origin.