Lymphocytic mastitis, also known as diabetic mastopathy or sclerosing lymphocytic lobulitis, is a benign clinicopathological entity that, in earlier studies, has been described as an uncommon cause of breast mass in adult females with long-standing insulin-dependent diabetes mellitus. Further studies have suggested an autoimmune aetiology owing to its association with other autoimmune diseases such as Hashimoto's thyroiditis. On clinical examination, mammography and ultrasound, this lesion may mimic breast carcinoma. The most common mammographic findings are ill-defined masses or asymmetric densities. Such lesions are often masked by dense glandular tissue, making mammographic evaluation difficult. Ultrasound often reveals the characteristic finding of an irregular, hypoechoic mass with marked posterior acoustic shadowing. We present a case of infiltrating ductal carcinoma with coexisting lymphocytic mastitis involving the right breast of a non-diabetic adult female who presented with complaints of a painless, hard palpable lump in her right breast for 2 months. Mammography and ultrasonography showed features of a malignant lesion that was subjected to fine needle aspiration cytology and tru-cut biopsy examination. Cytology revealed features suggestive of infiltrating ductal carcinoma in a background of severe inflammation and necrosis. Tru-cut biopsy showed features suggestive of lymphocytic mastitis. The patient underwent modified radical mastectomy of the right breast. Histopathological examination of right breast tissue revealed multifocal infiltrating ductal carcinoma, metastatic ipsilateral axillary lymph nodes, lymphovascular tumour emboli and tumour-free margins. The patient underwent adjuvant chemotherapy and radiotherapy. She is on hormone therapy with a selective oestrogen receptor modulator and is disease-free now.