Internal mammary lymph node biopsies were performed during radical mastectomy in 113 cases of invasive carcinoma of the breast. Metastases were found in 25 cases, 22% of the total. One hundred five of the cases received no further treatment unless cancer recurred and were observed for an average of 13.6 years. Local recurrence and 10-year mortality were significantly increased in the presence of internal mammary metastases, but it could not be demonstrated that the latter increased chest wall recurrence or decreased survival independently of axillary lymph node involvement. The survival of five patients with internal mammary metastases who were treated with postoperative irradiation or castration was not superior to that of 20 patients with untreated metastases. It was concluded that internal mammary metastases usually reflect generalized tumor spread and that local therapy directed at these nodes has a negligible influence upon cure of the disease. In that they reflect a poor prognosis, metastases at this site would justify systemic adjuvant chemotherapy.