Background: Due to their abundant vascular supply, the lungs are a usual metastatic site, with primary lung cancer presenting a low prevalence in dogs and cats. Among the primary pulmonary tumors afflicting dogs, lepidic carcinoma is the most common and can be classified by site of onset. Lepidic predominant adenocarcinoma is characterized by the proliferation of neoplastic cells along the pulmonary alveoli and pre-existing alveolar structures. Few reports of this specific tumor were found in the literature, so the objective of this study is to describe the clinical, pathological, and immunohistochemical findings of a dog with predominantly diffuse lepidic adenocarcinoma. Case: A 14-year-old, female, spayed, mixed-breed canine with a history of progressive weight loss, cough, respiratory distress, and pain while eating was brought in for a consultation. A presumptive diagnosis of pneumonia was suggested. However, in spite of treatment, the dog died, and a necropsy was performed. Upon gross examination, the lungs were diffusely enlarged, whitish, and stiffened with large and highly vascular nodules. Histopathological findings showed proliferation of neoplastic cells along the vascular walls and within the bronchioalveolar structures, with moderate fibrovascular stroma. The growth patterns resembled the pre-existing alveolar structures, with papillary protrusions into the alveolar lumen. After immunohistochemical evaluation, the definitive diagnosis was diffuse lepidic predominant adenocarcinoma. This tumor is uncommon and difficult to diagnose in the clinical veterinary routine, so it should be considered in the differential diagnosis of respiratory conditions in canines, especially in elderly animals with chronic clinical signs not responsive to antibiotic therapy. Complementary diagnostic examinations such as imaging, laboratory tests, and biopsy are indispensable to the early diagnosis of this lung neoplasm. Discussion: Due to its insidious character, primary lung cancer is underdiagnosed in veterinary medicine and is regularly an incidental x-ray finding. Affected dogs have no clinical signs, and this poses a diagnostic challenge. Primary lung cancer tends to be more prevalent in older and purebred animals. The most frequent clinical sign in dogs is a chronic nonresponsive cough, which can be misdiagnosed as another respiratory disease. Hematological findings, such as anemia, neutrophilia, and lymphopenia, can be related to tumor progression. Most reported cases of lepidic adenocarcinoma in dogs have a single pulmonary nodule, and the diffuse form is less common. In this case report, besides the presence of a single pulmonary nodule, there was also widespread involvement of the parenchyma. Histopathology was compatible with lepidic predominant adenocarcinoma, and immunohistochemistry was positive for pan-cytokeratin, confirming the epithelial origin of the tumor, and negative for TFF-1. Despite being a highly specific marker for lung cancer in dogs, TFF-1 has variable sensitivity among the different types of l...