Objective: To determine whether a rapid follicular growth pattern and a short follicular phase (SFP) during COH in IVF and ICSI cycles is associated with certain clinical or biochemical characteristics of such women, and whether it has an effect on the outcome of such cycles. Design: Retrospective controlled study. Materials/Methods: All IVF and ICSI cycles performed between 1–1-1998 and 31–12–2001 were reviewed. Cycles in which >=3 follicles had reached >=17 mm diameter after only nine days of COH and on which oocytes retrieval was performed on day 11 of cycle were identified as cycles with rapid follicular development and SFP. This was the study group of interest. Cycles in which COH was initiated on the same day as each of SFP cycles and in which oocytes retrieval was performed on cycle day 14 or 15 served as the control group. Cycles using donated oocytes were excluded. All women underwent pituitary down regulation using a short acting GnRH agonist followed by follicle stimulating hormone (FSH) or human menopausal gonadotrophin (HMG). Results: The study included 54 SFP cycles and 169 control cycles. Women age, type of infertility (primary or secondary) or order of IVF and ICSI cycles (first or repeat cycle) were not significantly different between SFP group and control group. Equally, basal FSH levels, treatment type (IVF or ICSI), total dose or type of used FSH or HMG were not significantly different. Male factor infertility was slightly more common in control group than in SFP group 92 (54%) vs. 20 (37%). There were no significant differences in other causes of infertility (tubal, anovulation, poor ovarian reserve, endometriosis and unexplained). The number of large size follicles (>=17mm) (5.4 vs. 5.1) or the total number of large and medium (14–16mm) size follicles (8.6 vs. 9.6) on ultrasound scan 48 hours prior to oocytes retrieval were not significantly different. There were less medium size follicles in SFP group (3.2 vs. 4.5, P <0.01). The number of retrieved oocytes (8.1 vs. 9.6, p = 0.03) and the number of fertilized oocytes (5.3 vs. 6.7, P = 0.04) were less in the SFP group. However, there was no significant difference in fertilization rate (70% vs. 69.7%), number of transferred embryos (2.1 vs. 2.05) and number of frozen embryos (2.1 vs. 2.8). The clinical pregnancy rate [18 (33%) vs. 63 (37%)], ectopic pregnancy, miscarriage or live birth rates were not significantly different between the two groups. Conclusions: In our study a SFP was not associated with advanced women’s age or higher basal FSH levels. The smaller number of medium size follicles in the SFP group suggests that a second cohort of follicles did not have a chance to complete its development. This reflected itself in a smaller number of retrieved and fertilised oocytes. Nevertheless, the numbers of frozen embryos, clinical pregnancy, live birth rates were not different and the clinical significance of these differences is therefore doubtful. A SFP seems therefore to have no detrimental effect on oocyte quality, endometrial development or embryo implantation. Although a SFP in the spontaneous cycle has been found by some to be associated with reduced fertility, this appeared not to be the case during COH for IVF and ICSI. During COH the outcome of cycles with SFP is comparable to cycles with ‘normal’ follicular phase length. Supported by: Centre for Reproductive Medicine, Coventry, UK.