Signet-ring cell carcinoma of the breast, first recognized in 1976, behaves aggressively and is associated with a poor prognosis. The tumor is a rare type of breast cancer and often metastasizes to the gastrointestinal tract and female genital tract. The authors report a case of a 48-year-old woman with bilateral signet-ring cell carcinoma of the breasts with multiple skeletal metastases and gastric metastasis. The breast cancer was depicted on a Tc-99m MIBI scintimammogram.