Breast metastasis from extra-mammary malignancy is rare. An incidence of 0.4-1.3% has been reported in the literature. The primary malignancies most commonly metastasizing to the breast are leukemia, lymphoma and malignant melanoma. We present a case of metastasis to the breast from a pulmonary adenocarcinoma, diagnosed concomitantly with the primary tumor. A 43-year-old female presented with dyspnea and a dry cough of 3 weeks' duration. A subsequent chest radiograph revealed a massive pleural effusion. Additionally, on physical examination, a poorly defined mass was noted in the upper outer quadrant of the right breast. The patient underwent bronchoscopy, simple right mastectomy and medical thoracoscopy. Following cytology, histology and immunohistochemistry, primary lung adenocarcinoma with metastasis to the breast and parietal pleura was diagnosed. Histologically, both the primary and metastatic anatomic sites demonstrated a micropapillary component, which has recently been recognized as an important prognostic factor. Although the patient received chemotherapy, she succumbed to her condition within 8 months. Accurate differentiation of metastasis from primary carcinoma is very important as the treatment and prognosis of the two differ significantly.