Objective: To compare the efficacy of standard and half doses of depot GnRH agonist triptorelin in pituitary down regulation, in a long protocol, during ovarian stimulation for IVF. Design: A prospective randomized study. Materials/Methods: 70 patients aged ≤38 years were randomized in two treatment groups. Pituitary desensitization was obtained in group 1 (35 patients) with half dose (1.87 mg) of triptorelin depot (Decapeptyl 3.75, IPSEN) in a single i.m. injection in the mid-luteal phase, and in group 2 (35 patients) with the standard full dose (3.75 mg) of the same GnRH agonist. At the onset of menses both groups received 4 ampoules (300 IU) of FSH (Metrodin HP 75, SERONO) daily for 2 days and 2 ampoules (150 IU) daily for 4 days; then the dose was adjusted according to the individual response. Results: No premature LH surge or luteinization occurred in both groups. LH levels were lower in the full dose group (group 2): on day 7 of stimulation the difference was not significant (1.1 ± 0.5 IU/L in group 1 vs 0.9 ± 0.5 in group 2: p = 0.09), but it became significant on the day of hCG administration (1.03 ± 0.47 IU/L vs 0.75 ± 0.3: p = 0.004). The number of FSH ampoules used was lower, but not significantly, in group 1 (44 ± 24 vs 51 ± 19). Estradiol levels, the number of follicles and oocytes collected and fertilised, and the number of embryos obtained or transferred were similar in the 2 groups. There was a trend toward higher pregnancy rate per transfer in group 1 (40.6% vs 31.2%, not significant). No significant differences were found in terms of implantation rate (20.3% vs 14.5%) and abortion rate (15.4% vs 10%). Conclusions: Half dose (1.87 mg) of depot triptorelin can be successfully used in ovarian stimulation for IVF or ICSI, with no impairment in quality of ovarian stimulation and overall treatment results. LH levels confirm the lower degree of pituitary desensitization achieved by the half dose of triptorelin. A higher number of patients should clarify if this lower desensitization will improve the outcome of IVF. Anyway, considering the high costs of ART treatments, a reduction of the amount of drugs required for stimulation might be a financial advantage for patients.