We describe a case of simultaneous development of a medullary carcinoma of the breast and multicentric reticulohistiocytosis (MRH) with identical characteristic histological lesions being found not only in the skin and synovium, but also in close proximity to the breast cancer as well as the connective tissue surrounding the tumor involved axillary lymph nodes. Successful treatment of the primary malignancy resulted in complete resolution of the MRH over 4 months. A close temporal relationship existed between the development and clinical remission of the malignancy and the course of the arthritis and skin lesions. Characteristics of this rare syndrome with particular reference to its association with malignancy are reviewed.