Tobey Jones, a 13 year-old male castrated Domestic Shorthair cat, presented to the Emergency Service at Cornell University Hospital for Animals for evaluation of dypsnea and acute onset, episodic open-mouth breathing. The cat's history included many months of inappetance, weight loss, and episodic, non-productive coughing. Physical examination revealed generalized poor body condition (body score: II/V), multiple skin nodules, and increased lung sounds. Cytology of multiple skin nodules and of pleural fluid showed a predominance of round cells with abundant cytoplasm. The morphologic of the round cells, combined with a history of pulmonary disease, lead to a cytologic diagnosis of malignant histiocytosis (MH). Malignant histiocytosis (also termed disseminated histiocytic sarcoma) is a type of round cell tumor composed of antigen presenting cells of dendritic cell origin. Histiocytic disease can be broadly classified as either a reactive or neoplastic process, distinguished by clinical behavior, histopathologic features, and immunohistochemical markers. Malignant histiocytosis is an aggressive, systemic, neoplastic condition that is uncommon in the dog, although Bernese Mountain Dogs, Rottweilers, and Retrievers are overrepresented. It is rare in the cat. In both species, MH has a rapidly progressing clinical course and is poorly responsive to chemotherapy. Its prognosis is grave. This paper will focus on the classification of histiocytic disease, and what is known to date about MH in feline patients.