Introduction: Phylloides tumours (PTs) are rare fibroepithelial neoplasms that account for 0.3–0.9% of all breast tumours. These tumours typically occur in women aged 30–70 years. The occurrence of these tumours in older children and adolescents poses particular diagnostic and therapeutic problems. However, early diagnosis is mandatory because although most of the cases of PTs in children are benign, the borderline and malignant cases with potential negative outcomes cannot be excluded. Case presentation: A 12-year-old girl presented at the Paediatric Emergency Department for hyperaemia and warmth of the left breast that occurred a few days prior without fever. The girl experienced menarche 8 months previously. She experienced no previous trauma and she had no family history of breast cancer. On physical examination, the left breast was painful, enlarged and tender. The overlying skin was erythematous and warm. A breast ultrasonography (US) revealed a large mass with features of an abscess, including a hyperechoic wall, scattered internal echoes and hypoechoic peripheral lacunae of apparent colliquative nature. After 4 days of unsuccessful antibiotic therapy, surgical drainage was performed due to the suspicion of a mammary abscess. At the surgical incision site, the lesion was not-well circumscribed and lacked a capsule. In addition, purulent material was not detected. Histological examination revealed that the tissue alterations were compatible with benign PT. With this diagnosis, the girl underwent definitive surgical removal of the lesion. The postoperative period passed without negative events. An US performed 6 months later revealed that no new mass was present at this time, suggesting no recurrence of the tumour. Conclusion: This case shows that in the presence of a clinical picture suggesting the inflammation of the breast in adolescent females, PT should be considered as a possible diagnosis and US-guided core biopsy should be considered to confirm this suspicion. Thereafter, when surgical excision is performed, particular attention must be paid to both the preservation of all the normal breast parenchyma and future aesthetic problems.