Bear, an 8-year-old castrated male Bernese Mountain dog, presented to Cornell University Hospital for Animals on June 9, 2009 for lethargy and wheezing for two days. Bear was febrile and in respiratory distress, with harsh lung sounds in all fields. Radiography revealed tracheobronchial and cranial mediastinal lymphadenopathy, as well as a nodular pattern throughout the lungs. Abdominal ultrasound was relatively unremarkable, however fine needle aspirates of the spleen and liver revealed marked histiocytic infiltrate of both organs without evidence of malignancy. A similar infiltrate was present in his mandibular salivary glands and lymph nodes days later. Histiocytosis from pyogranulomatous infection was ruled out after negative Bartonella PCR, 4DX test, and acid-fast stain. Bear’s histiocytic infiltrate did not show typical signs of malignancy, however Bear had the clinical features of an aggressive, malignant disease. A presumptive diagnosis of disseminated histiocytic sarcoma, an inherited disease in the breed, was made.