An 8-year-old Thoroughbred mare presented with acute bilateral epistaxis. The mare showed no clinical signs other than cough and profuse bilateral epistaxis. Antibiotic therapy was instituted and a transtracheal wash was collected. Large numbers of inflammatory neutrophils and gram-positive cocci were reported in the tracheal wash. The antibiotic therapy was continued for 7 days at which time the mare appeared clinically normal. The mare was successfully bred and was confirmed pregnant. She was referred to a hospital when the epistaxis recurred. She was treated with another antibiotic, and thoracic radiographs were also taken which revealed a space occupying lesion. The mare foaled a live foal. She developed bilateral thickening of her limbs. The swellings were of boney consistency and affected all four legs, particularly the forelimbs. A diagnosis of pulmonary hypertrophic osteopathy was made. The mare was again sent for further investigation. The thoracic mass had increased in size from the first examination. A biopsy was collected by endoscopy which confirmed a neoplastic process. After deterioration in her condition, the mare was euthanized on humane grounds. An autopsy report revealed that the mare had approximately 80% of the left lung replaced by neoplastic tissue. The right lung contained many small nodular masses. There was an associated cardiomegaly. In this case, the development of periosteal proliferation in multiple metaphyses was secondary to an intrathoracic neoplastic mass.