BACKGROUND: Hypophosphatasia (HPP) is a rare inherited metabolic disorder caused by mutations in the ALPL gene, which encodes tissue nonspecific alkaline phosphatase. The severity of HPP is widely diverse from the perinatal form to the adult mild form. The former represents the most severe form and was earlier associated with high mortality due to pneumonia which was caused by severe hypomineralization of the bones-such as chest deformity and fractured ribs-and muscle weakness. Enzyme replacement therapy using asfotase alfa (AA) was approved in 2015 in Japan for treating patients with HPP and has improved their pulmonary function and life prognosis. There are several practical and ethical challenges related to using orphan drugs for a rare disorder in a publicly funded healthcare system. Sharing experiences about their application is essential towards formulating guidelines to assist clinicians with decisions about their initiation and withdrawal. We report the details of AA experience in ten cases of pediatric-onset HPP in nine families from January 2015 to November 2019 (median [interquartile range] age 11.0 [7.6-12.5] years; 60% male). This is a study of a single-center cohort describing the clinical course of patients with HPP, mainly consisting of the mild childhood form of HPP, treated with AA in Japan. RESULTS: One case of perinatal form of HPP, two cases of benign prenatal form, and seven cases of childhood form were observed. The most common symptom at onset was pain. All patients had low serum alkaline phosphatase levels as compared to the age-matched reference range before the commencement of AA. All HPP patients seem to have responded to AA treatment, as evidenced by pain alleviation, increased height standard deviation, improvement in respiratory condition and 6-min walk test result improvement, disappearance of kidney calcification, alleviation of fatigue, and/or increases in bone mineralization. There were no serious adverse events, but all patients had an injection site reaction and skin changes at the injection sites. Genetic analysis showed that eight out of ten patients had compound heterozygosity. CONCLUSIONS: AA may be effective in patients with mild to severe pediatric-onset forms of HPP.