The application of LH-RH provides means to describe the functional state of the hypothalamo-pituitary-gonadal axis and has helped to understand the regulation of gonadotropin secretion both under normal and pathological circumstances. At present, diagnostic LH-RH application appears to be most useful in patients with delayed puberty, where a biphasic response during an infusion of LH-RH excludes hypogonadotropic hypogonadism at an early clinical stage. An enhanced LH-RH induced response following exogenous intermittent LH-RH administration identifies the subgroup of hypogonadotropic patients who are suitable for subsequent LH-RH substitution therapy. In addition, subtle derangements of gonadotropin secretion have been described in a score of clinical conditions including the endocrine disorders discussed in this review. However, interesting as these observations may be theoretically, their practical value is often limited since a multitude of interfering factors, both exogenous and endogenous, results in a great variability in the obtained results, thus limiting the diagnostic value of the test in a specific patient. As of recently the clinical use of LH-RH includes its therapeutic application to replace low endogenous levels of LH-RH, whereas long-acting analogues of the natural decapeptide are employed to suppress gonadotropin secretion in an increasing field of endocrine disorders.