Background: Gonadotropin-releasing hormone agonists (GnRHa) represent the gold-standard treatment for central precocious puberty (CPP). In CPP children, GnRHa treatment slows bone age progression and preserves adult height (Ht) by suppressing sexual steroid secretion. In some patients, however, GnRHa induce an inappropriate growth deceleration impairing Ht outcome. Furthermore, slowly progressive CPP (spCPP) forms were reported which do not need GnRHa treatment. Methods: We evaluated the growth outcome of 26 spCPP girls treated with triptorelin (TR) and 21 with leuprorelin acetate (LA) for 36.5 ± 0.7 months. Results: GnRHa treatment induced a progressive growth deceleration in both spCPP groups. No difference in bone maturation was detected (p > 0.05; TR vs. LA group), however compared to LA, TR treatment resulted in significantly higher Ht after 24 months (p < 0.05; LA vs. TR group). Although target height (TH) standard deviation score (SDS) and predicted adult height (PAH)-SDS at diagnosis were similar in both spCPP groups (p > 0.05; LA vs. TR group), final height (FH-SDS) was lower in LA-treated subjects (p < 0.05; LA vs. TR group). In both spCPP groups, FH-SDS was significantly lower than TH-SDS (p < 0.001) but not lower than PAH-SDS at diagnosis (p > 0.05). Ht-SDS correlated with 17β-estradiol (E2) blood levels in both spCPP groups (p < 0.0001) throughout GnRHa treatment, and E2 values were higher in the TR- than in the LA-treated patients during the 12 months after GnRHa administration (p < 0.05; LA vs. TR group). GnRHa-induced E2 secretion and Ht-SDS at GnRHa withdrawal correlated positively with FH (p < 0.01 and p < 0.001, respectively). Conclusions: The effectiveness of GnRHa treatment in improving FH in spCPP girls was doubtful.