We report a 51-year old woman who suffered 2 atypical subtrochanteric femoral fractures (AFFs). She had a history of several metatarsal fractures. She had a normal bone densitometry. An adult form of hypophosphatasia (HPP) was diagnosed from low serum alkaline phosphatase (ALP), and tissue nonspecific isoenzyme of ALP (TNSALP) mutation analysis revealing 2 heterozygous mutations: c.299C>T (p. T100M) and c.571G>A (p. E191K). Low ALP is the hallmark of the diagnosis of HPP; which is associated in adults with premature loss of deciduous teeth, recurrent metatarsal stress fractures, and joints and tendons disorders. The incidence of AFFs in the population is 5.9 per 100,000 person-years. Physicians and patients with bone fragility must pay attention to prodromal pain, which require urgent radiographic evaluation of both femurs. Rheumatoid arthritis, use of glucocorticoids, and proton pump inhibitors have been associated with an excess risk of AFFs. Healthy subjects carrying a TNSALP mutation with low ALP value may be exposed to develop AFF spontaneously or while receiving potent anti-resorptive drugs. Low ALP must be checked as a cause of bone fragility.