Castleman's disease is a rare lymph node pathology characterized by angiofollicular hyperplasia. There are two forms of the disease: localized and systemic, with different features, symptoms and prognosis. Three are the histological types of disease: plasma cell, hyaline-vascular and mixed variants. We report the case of a 65-year-old female affected by localized plasma cell variant of Castleman's disease. The singularity of our case lies in its localization on the breast and monoclonal plasma cell proliferation inside the nodule.