A case of ductal adenoma of the breast in a 64-year-old Japanese woman is reported. The patient presented with a well-defined, 0.9 cm, firm mass in the right breast. Histological examination of the excised tumor revealed small and medium-sized ductules forming a nodular pattern partly surrounded by a thin layer of mature collagenous fibers. A few hyalinized fibers were also observed among the ductules, which had a clearly defined basement membrane, diagnosed as ductal adenoma of the breast. Determination of steroid hormone receptor by a dextran-coated charcoal method revealed that the ductal adenoma contained high concentration of estrogen receptor (ER) and progesterone receptor (PgR) (280 and 105 fmol/mg protein, respectively). The patient had a previous history of invasive ductal carcinoma of the left breast and had undergone a modified radical mastectomy 4 years earlier. The contralateral breast cancer was also positive for both ER and PgR (350 and 78 fmol/mg protein, respectively).