SummaryWe report the case of a 55-year-old woman who presented to the emergency department having woken from sleep with right sided thigh swelling. Pelvic radiographs revealed bilateral atypical subtrochanteric femoral fractures (ASFFs). In the two years leading up to this admission, the patient had experienced gradually increasing pain and weakness in her legs which had resulted in a decrease in her mobility from fully mobile to bed-bound. During this time a neurologist had organised a magnetic-resonance imaging (MRI) scan of the brain and spine which was normal. There was no history of bisphosphonate (BP) use. Historical and admission blood tests revealed a persistently low serum alkaline phosphatase (ALP), with all other results within normal limits. The patient was treated with intramedullary nailing of both femurs and histological analysis of bone reamings were characteristic of hypophosphatasia (HPP). The patient was independently mobilising with a walking frame on discharge. Subsequent genetic testing revealed bi-allelic pathogenic variants in the TNSALP gene: c.526G>A, p.(Ala176Thr) and c.1171C>T, p.(Arg391Cys). HPP is an inborn error in metabolism caused by mutation in the gene coding for tissue non-specific alkaline phosphatase (TNSALP), resulting in a decrease in serum ALP concentrations. The age at which it presents which can vary from childhood to middle age, with symptoms ranging from perinatal death to late-onset osteomalacia. In those patients who survive to adulthood, there is a predisposition to fractures, including ASFFs. Treatment with asfotase alfa (a bone-targeted, recombinant human TNSALP) has been approved for perinatal, infantile and paediatric-onset hypophosphatasia.This case emphasises the importance of viewing persistent low ALP as a 'red flag' in patients presenting with musculoskeletal symptoms. Timely diagnosis and treatment of HPP can reduce the risk of serious complications, such as those experienced by this patient.