Background: In the routine work-up of suspect breast lesions, ultrasound-controlled core needle biopsy (CNB) is the most common tool to acquire tissue for histopathologic analysis in a safe, quick and convenient way. Complications are generally rare. The most common complications are hematoma and infection, each with less than 1 in 1000 cases. Case Report: Here, we present a case of a 48-year-old patient who underwent CNB for several lesions that were assessed as Breast Imaging Report and Data System (BI-RADS) IV in breast ultrasound and mammography. In the past, she had had 2 bilateral breast reduction surgeries and 1 open biopsy of a fibroadenoma. Histology revealed a phyllodes tumor. Following this, mastitis occurred which was resistant to common conservative measurements such as intravenous antibiotics over months. Finally, mastectomy was performed, followed by adequate wound healing. Conclusions: In the presented case, the prolonged course of breast infection after CNB was not as expected. If this occurs, conservative treatment with antibiotics can be initiated. Possible additional risk factors such as diabetes mellitus, steroid therapy, or immunosuppression should be identified. However, in case of missing recovery, wide surgical excision is recommended.