Six patients with metastatic carcinoma of the breast underwent mediastinoscopy to obtain tissue for estrogen receptor analysis and pathologic confirmation of metastatic tumor. The indication for mediastioxcopy was an abnormal mediastinal accumulation of gallium in five patients, only two of whom had an abnormality noted on tomography. All six patients had tumor recovered by mediastinoscopy as demonstrated by pathologic examination and/or estrogen receptor assay, Therefore, the diagnosis of mediastinal metastases in breast cancer may be suggested by either chest roentgenograms, mediastinal tomography, or gallium scintigraphy. Mediastinoscopy is a safe, effective procedure capable of establishing the diagnosis of metastatic carcinoma of the breast and of procuring sufficient tissue for estrogen receptor analysis in patients without more readily accessible sites of metastases.