This study was designed to evaluate whether a single blood sample drawn after the home injection of a long-acting gonadotropin-releasing hormone (GnRH) agonist (GnRHa) in patients treated for central precocious puberty (CPP) could be a more simple and inexpensive test with respect to the conventional GnRH stimulating test in assessing adequate suppression of the pituitary-gonadal axis. The response to the first therapeutic injection of the GnRHa triptorelin was studied in 14 newly diagnosed untreated females with CPP. The results were compared with the response that the same patients had to the conventional GnRH stimulation test performed at the time of diagnosis. A significant increase in LH, FSH and E<sub>2</sub> levels was observed 12 h after the triptorelin intramuscular injection; serum peak values of LH (70.3 ± 58.5 IU/l), FSH (44.2 ± 21.7 IU/l) and E<sub>2</sub> (489.7 ± 263.9 pmol/l) were significantly greater than those obtained with the conventional GnRH test (LH 31.4 ± 21.7, p = 0.002; FSH 19.8 ± 10.7, p = 0.001; E<sub>2</sub> 83.3 ± 25, p < 0.001). In particular, the E<sub>2</sub> response, 12 h after triptorelin injection, was clearly consistent with gonadal activation compared to the modest E<sub>2</sub> increase in response to the GnRH test. Thereafter 22 girls who were already being treated with triptorelin for CPP were evaluated to see whether a single blood sample drawn 12 h after the therapeutic home injection of GnRHa could be informative in assessing adequate suppression of the pituitary-gonadal axis. This response was also compared to the conventional GnRH stimulation test performed 2 days before the therapeutic triptorelin injection. In 7 girls with evidence of pubertal progression, the E<sub>2</sub> response following the GnRHa injection (136.3 ± 44.4 pmol/l) was significantly higher with respect to the response after the GnRH stimulation test (73.0 ± 0.0; p < 0.02) indicating an inadequate suppression of the pituitary-gonadal axis. The present data suggest that a single blood sample drawn 12 h after the therapeutic home administration of triptorelin provides a simple, comfortable and inexpensive means of monitoring pituitary as well as gonadal function in girls treated for CPP.