A case of myxoma of the breast was reported. A 25-year-old woman showed a lump in the left breast which was suddenly enlarged after the delivery of a baby. A tumor measuring 73.53cm was located in the upper outer quadrant of the breast. Fine needle aspiration cytology revealed scattered benign spindle shaped mesenchymal cells with abundant myxoid material. On excision, the tumor was partly solid, partly cystic and multilocular. Each nodule consisted of an abundant mucoid material with a few spindle or stellate mesenchymal cells. Pleomorphism and mitosis were absent. The mesenchymal cells showed immunoreactivities for S-100 protein, smooth muscle actin, and CD34. The myxoid stroma was positively stained with alcian blue and mucicarmine. These findings suggest that constituent cells derive from totipotential primitive mesenchymal cells.