We report an extremely rare case of the colocalization of a tubular adenoma and an invasive ductal carcinoma occurring in a 55-year-old woman. Following radiographical evaluation, fine-needle aspiration cytology of the left breast tumor was undertaken. Because cytological examination revealed malignancy, a partial mastectomy was performed. Histologically, the tumor (measuring 1.7 x 1.3 cm) comprised two distinct parts: tubular adenoma and invasive ductal carcinoma. The invasive ductal carcinoma showed a solid pattern, high nuclear and structural atypia and frequent mitotic figures, while the tubular adenoma consisted of a proliferation of small ducts lined by single layers of epithelial and myoepithelial cells with bland nuclei and inconspicuous nucleoli. The histological boundary was clearly defined between the tubular adenoma and the invasive ductal carcinoma, and between the tubular adenoma and the surrounding breast tissue. The current case might be a collision between separate tubular adenoma and invasive ductal carcinoma, but the malignant transformation of a tubular adenoma cannot be ruled out. Both the long-term observation of this case and analysis of more cases may enable us to determine the histological characteristics and clinical significance of invasive ductal carcinoma associated with tubular adenoma.