Rosai-Dorfman disease or sinus histiocytosis with massive lymphadenopathy is a rare proliferative histiocytic disorder of the lymph nodes. Extranodal involvement occurs in a considerable number of cases; however, involvement of the breast is very rare, and it is even rarer for the lesion to be localized in the breast alone without affecting any other sites. This report describes the case of a 50-year-old Brazilian woman with a lump confined to her left breast that had clinical and radiological characteristics indistinguishable from cancer. The proliferation of histiocytes, displaying lymphophagocytosis and an S-100 protein immunophenotype on a core biopsy of the lesion, led to a diagnosis of Rosai-Dorfman disease and permitted conservative therapy. Recognition of this rare condition, when occurring at an unexpected site such as the breast, is difficult, and the correct diagnosis is important prior to therapeutic management.