Abstract Disclosure: V.M. Figueredo: None. T. Seeherunvong: None. Background: Precocious puberty is typically associated with a speed up in growth, risk of premature growth plate closure and reduced adult height. Serum alkaline phosphatase (ALP) is a biochemical indicator of bone turnover and studies have shown greater serum ALP levels in pubertal individuals compared to prepubertal controls. We examined serum ALP levels among children with various form of early puberty. We investigated whether serum ALP levels were significantly different between children with premature thelarche or premature adrenarche compared to those with central precocious puberty (CPP). Methods: We conducted a retrospective chart review of patients with early pubertal development attending a pediatric endocrinology clinic between 2017 and 2023. Information on height, weight, serum ALP levels and bone age was obtained. Patients were considered to have CPP based on the results on a GnRH test. Patients with diagnosis of premature adrenarche or premature thelarche were included as controls. This study was approved by Institutional Review Board (IRB). Analysis of data was performed using IBM SPSS Statistics version 29.0.1.0 (171). Results: Among 72 patients included in the study, 22 patients had progressive CPP whereas 50 patients had premature adrenarche or premature thelarche. The prevalence of abnormally elevated serum ALP levels was greater among patients with CPP (22.7%) than among patients with premature adrenarche or premature thelarche (6%). Using logistic binomial regression analysis, serum ALP levels were significantly elevated (p=0.042) in subjects with progressive CPP compared to patients with premature adrenarche or premature thelarche. Both groups had similar BMI values. BMI z-scores and advanced bone age were positively correlated. BMI (p=0.08) and advanced bone age were not significantly different (p=0.12) in the group with CPP compared with the group comprising patients with either premature adrenarche or premature thelarche. Conclusion: These findings suggest that serum ALP levels are helpful as a tool to help distinguish CPP from premature adrenarche or premature thelarche in the evaluation of patients with initial early pubertal development. Presentation: 6/1/2024