Background: A test involving 100 μg of intravenous gonadotropin-releasing hormone (GnRH) is a gold standard for confirming the diagnosis of central precocious puberty (CPP). However, intravenous GnRH for testing is commercially limited. Objective: To develop subcutaneous GnRH agonist (GnRH-A) testing and define a peak luteinizing hormone (LH) cutoff value in diagnosing CPP. Methods: A retrospective study of 101 girls with sexual precocity was undertaken. All girls underwent 100 μg subcutaneous GnRH-A (triptorelin) testing. Blood samples before and 30, 60, 90 and 120 min after GnRH-A injection were analyzed for LH and follicle-stimulating hormone levels. Criteria for diagnosing CPP include accelerated height, advanced bone age and pubertal-sized uterus and ovaries. Results: Fifty-five girls were documented as having CPP. The remaining 46 girls were diagnosed with premature thelarche (PT). Peak LH concentration in the CPP group was significantly greater than that of the PT group with a median (range) of 10.0 IU/l (2.93–65.39) and 3.04 IU/l (0.19–8.82), respectively. Peak LH was achieved within 60 min following GnRH-A injection. Peak LH of 6 IU/l provided the most appropriate cutoff level in diagnosing CPP with a sensitivity of 89.1% and a specificity of 91.3%. Conclusion: Subcutaneous GnRH-A can be used as an alternative to confirm the diagnosis of CPP.