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• An increase in the expression or activity of ALP suggests osteoblast differentiation. • ALP activity can be an important indicator of metabolic bone diseases. • A percentile curve for ALP activity per month in infants was created. • This curve is a reference for diseases related to abnormal ALP activity in infants. Alkaline phosphatase (ALP) is a ubiquitous enzyme in humans that can be used for diagnosing childhood diseases. Infants have the highest rapid growth rate and are susceptible to metabolic bone diseases. In infants, ALP activities exhibit significant month-wise variations, and authoritative standards are lacking. The present study aimed to provide a reference for the diagnosis of diseases related to abnormal ALP activities in infants. This study included 24,618 samples collected from infants aged 0–12 months from three medical centers in Chongqing, China. Samples of infants diagnosed with diseases that may affect ALP activity have been exclude. ALP activity was analyzed using an automatic biochemical analyzer. A percentile curve for ALP activity in male and female infants was constructed using MATLAB, and the skewness-median-coefficient of variation method was employed for curve fitting. ALP activity in male and female infants peaked at 0–4 months; the peak appeared at 1–2 months and declined gradually thereafter. After 4–5 months of age, the ALP activities declined further, with the lowest values observed at 11–12 months of age. A comparison between the data from this study and a those from a published German study indicates that Chinese infants exhibited peak ALP activity later and subsequent decline greater than German infants. A percentile curve was constructed for month-wise ALP activity in male and female infants, which could provide a reference for diagnosing diseases related to abnormal ALP activity in infants. [ABSTRACT FROM AUTHOR]