Giant benign mesenchymoma of the breast. The term mesenchymoma refers to a group of mixed tumors that are composed of two or more mesenchymal elements, excluding fibrous tissue, not ordinarily found together within the same tumor. Mesenchymomas occur most commonly in the renal and perirenal regions with rare occurrence in the breast. We describe what to our knowledge is the first report of a giant benign mesenchymoma of the breast. The clinical presentation, course, and treatment of a patient with this condition is discussed. Clinicians should be aware that benign mesenchymomas may involve the breast and simulate a malignant breast neoplasm. 