It is necessary not only to perform laboratory tests for the correct diagnosis of children with precocious puberty, but also to monitor laboratory tests to ensure adequacy of therapy. Careful laboratory testing is a requisite in the differentiation of central from peripheral precocious puberty. It is also required to determine whether the patient who presents with early physical changes of pubertal development with peripheral precocious puberty has evidence of pubertal hormonal secretion. The most useful single test is gonadotropin-releasing hormone (gonadorelin) stimulation of luteinizing hormone release. The same stimulation test is also indicated to verify the adequacy of suppression of gonadotropin secretion among patients with central precocious puberty who are being treated with gonadotropin-releasing hormone analogue. It is necessary to know which gonadotropin assay is being used and the range of normal levels to correctly interpret the tests.