Ovine pulmonary adenocarcinoma (OPA) is a contagious and transmissible lung cancer of sheepresembling human bronchiolo-alveolar carcinoma. In the present study, lungs of 9400 sheep slaughtered inFars province, Iran were examined morphopathologically. The OPA was diagnosed in the lungs of 21(0.22%) out of 9400 sheep. Frequency of involvement of different lobes in the affected lungs was includedapical lobe (62%), cardiac lobe (33%), middle lobe (33%), diaphragmatic lobe (62%) and accessory lobe(5%). In 12 sheep, the classical form observed as firm, white to grayish coalescing masses mostly in thecranio-ventral lobes associated with wet cut surface and frothy fluid in the airways. In 9 sheep, atypicallesions observed as small clearly demarcated nodules mostly in diaphragmatic lobes associated with dry cutsurface and minor fluid in the airways. Histopathological changes were almost similar in the two forms. Theyconsisted of an acinar or papillary growth of neoplastic cells in the alveoli and polypoid proliferation ofbronchiolar epithelium. There were variable amounts of connective tissue, myxomatous foci and infiltrationof lymphocytes and plasma cells in the interstitial tissue of the affected alveoli. No metastatic lesion wasobserved in the lymph nodes. The findings of this study show that atypical and classical forms representdifferent stages or manifestations of a single disease spectrum.