Juvenile gigantomastia in the paediatric age group is an extremely rare condition characterised by massive breast enlargement in the peripubertal phase in girls. We report the case of a 10-year-old girl with learning difficulties who presented with rapid, asymmetrical, massive breast hypertrophy. Her bilateral gigantomastia was complicated by significant pain, skin breakdown and hypercalcaemia. Biochemical investigations showed elevated PTH-rP (parathyroid hormone-related-protein) as the driver for the hypercalcaemia. Medical management included hyperhydration, furosemide, intravenous antibiotics, chlorpheniramine, opiates and hormonal suppressive treatment using GnRH (gonadotrophin-releasing hormone) agonist therapy with no response, with the addition of tamoxifen which achieved minimal response. Complex clinical decisions around definitive surgery required a patient-centred and multidisciplinary approach. Bilateral simple mastectomy (removal of breast tissue, nipple, areola and skin) was performed with no post-operative complications, achieving prompt resolution of hypercalcaemia improvement in behavioural symptoms with no breast tissue recurrence at 20 months. Rare scenarios of breast tissue overgrowth are problematic to manage in children with additional complex health needs, but also provide rare insights to the pathophysiology of calcium metabolism.