Objective: The objective of this study is to investigate whether the degree of down-regulation using GnRH-agonists is associated with pregnancy outcomes. Study design: This retrospective analysis was performed on 2708 cycles from 2514 patients undergoing down-regulation with the luteal phase long protocol. The serum oestradiol (E2D) and luteinising hormone (LHD) levels, the diameter of the largest follicle (DLFD) and the endometrial-thickness (ENTD) after down-regulation were used to evaluate the degree of down-regulation. One-way analysis of variance with the Bonferroni adjustment, the chi-square test and multivariate logistic regression analyses were used for the statistical analysis. Results: The cumulative clinical pregnancy rates (CCPR) and the cumulative live birth rates (CLBR) were higher in the cycles with E2D<30 pg/ml (63.7%, OR = 1.405 (1.055-1.870) and 56.8%, OR = 1.372 (1.039-1.813)) and 30-55pg/ml (66.8%, OR = 1.439 (1.104-1.874) and 59.8%, OR = 1.397 (1.080-1.806)) than in those with E2D>55 pg/ml (62.8% and 54.7%). There was a trend towards lower CCPRs and CLBRs in the cycles with DLFD>10mm or ENTD >= 6 mm; however, this difference was not significant. Conclusion: The degree of down-regulation is associated with ovarian response, pregnancy, and live birth. We propose the following criteria for optimal down-regulation: E2D 30-55 pg/ml, ENTD<6 mm, and no apparent ovarian activity.