Histiocytes are immune cells that originate in the bone marrow and complete maturation in the organs. The first breed of dog associated with malignancies of this cell type was the Bernese Mountain Dog in the late 1980s. Since then, a variety of breeds appear to be predisposed to the development of this form of cancer, including flat-coated, golden, and Labrador retrievers, as well as rottweilers. Signs associated with this cancer are often nonspecific, as it typically affects multiple organs (including lungs, liver, spleen, muscle, and bone marrow) at the same time. Occasionally, an external lump or mass can be noted as well. Signs of malaise, weight loss, cough and inappetance are common. Determining the extent of disease via blood tests, radiographs and ultrasound is important in predicting prognosis. Often special staining of the cells (i.e. immunohistochemistry) is required for definitive diagnosis, as this form of cancer can appear microscopically like several other malignancies. Unfortunately, MH is a rapidly progressive cancer. While some dogs respond temporarily to chemotherapy with improved quality of life, oncologists continue to search for new therapeutic options to combat this aggressive disease. A similar, but very rare form of this disease occurs in people. It is the hope of the CCOGC that with further study, we will advance our understanding of this deadly disease both in dogs as well as in people.