Large benign lesions of the breasts are rare in children. We present a case of a 35 cm mass, weighing 2.7 kg in a 13-year-old girl with small developing breasts. Despite the enormity of the lesion, the patient managed to keep it concealed from her parents for 8 months. While initially suspicious of sarcoma a diagnosis of pseudoangiomatous stromal hyperplasia was suggested radiologically and confirmed histologically. Excision with reduction mammoplasty was performed, care taken not to disrupt the remaining breast tissue to facilitate future breast development. 18 months on, the cosmetic appearance of the breasts is good, with healthy underlying breast tissue developing. To the best of our knowledge this case is the largest documented breast tumour of this type in a patient of this age and illustrates the challenge of treating such tumours in the developing breast.