Simple SummaryTumors are a very rare condition in cattle. Moreover, they are poorly described in the literature, especially in bovine species. The aim of this study was to accurately describe a procedure leading to the diagnosis of thoracic sarcoma in a beef cow. The procedure started from the initial clinical signs motivating specific investigations (blood biochemistry, cytology, thoracic ultrasonography, and ultrasound-guided biopsy) and resulted in interesting laboratory findings with histology and targeted immunochemistry. The concomitant support treatment is also thoroughly provided in the manuscript. This study may offer valuable support for veterinary practitioners in future similar cases.AbstractTumors in cows are not frequently reported in the literature. They often represent unusual findings in live animals and are incidental at slaughter with rare positive therapeutic outcomes for farmers. A 9-year-old beef cow was referred to the hospital of ruminants of the National Veterinary School of Toulouse, France. The cow started to become sick 10 days prior, and major symptoms were anorexia, arched back, tachycardia, and tachypnea associated with significantly attenuated cardiac and pulmonary sounds upon right-sided auscultation. After specific investigations, a thoracic sarcoma associated with unilateral empyema was diagnosed. The empyema was treated, and supportive treatment was only performed for the tumor. Although the sarcoma remained, clinical improvement was significant, and the cow went back to her farm of origin. After the end of the withdrawal period, the cow recovered clinically but was culled by the owners for economic reasons. The present case report offers a continuum from the initial clinical signs motivating specific investigations to interesting laboratory findings, which were confirmed post-mortem.