A 70-year-old woman with breast cancer treated with hormonal therapy had progressive shortness of breath for one month. Chest radiograph and computed tomography showed mild interstitial changes, but could not account for her respiratory failure. Lymphangitic carcinomatosis, drug-induced pneumonitis, idiopathic interstitial pneumonitis, opportunistic infection, and pulmonary edema were considered in the differential diagnosis of the CT findings. A perfusion scan revealed numerous small subsegmental perfusion defects in both lung fields. Bronchoalveolar lavage fluid (BALF) contained some cancer cells, suggesting lymphangitic carcinomatosis. Transbronchial biopsy (TBLB) specimen showed tumor emboli in small pulmonary arteries. Immunohistochemical findings of TBLB specimen were consistent with breast cancer cells. A diagnosis of tumor microembolism caused by breast cancer metastasis was made. Antemortem diagnosis of tumor microembolism is very difficult. Here, we report a case of tumor microembolism diagnosed by perfusion scan and TBLB.