Objective To assess the impact of elevated early follicular progesterone (P) levels in gonadotropin-releasing hormone (GnRH) antagonist cycles on clinical outcome using prospective data in combination with a systematic review and meta-analysis. Design Nested study within a multicenter randomized controlled trial and a systematic review and meta-analysis. Setting Reproductive medicine center in an university hospital. Patient(s) 158 in vitro fertilization/intracytoplasmic sperm injection (IVF-ICSI) patients. Intervention(s) Recombinant follicle-stimulating hormone (FSH) (150-225 IU) administered daily from cycle day 2 onward; GnRH antagonist treatment randomly started on cycle day 2 or 6; assignment into two groups according to P level on cycle day 2: normal or elevated (>4.77 nmol/L or >1.5 ng/mL, respectively). Main Outcome Measure(s) Ongoing pregnancy rate (OPR) per started cycle. Result(s) The incidence of elevated P was 13.3%. A non-statistically- significant difference in OPR was present between the normal and elevated P groups (27.0% vs. 19.0%). No differential impact of early or late GnRH antagonist initiation on the effect of elevated or normal P on OPR was observed. A systematic search of Medline and EMBASE from 1972-2013 was performed to identify studies analyzing elevated early P levels in GnRH antagonists. The meta-analysis (n = 1,052) demonstrated that elevated P levels statistically significantly decreased the OPR with 15% (95% CI -23, -7 %). Heterogeneity across the studies, presumably based on varying protocols, may have modulated the effect of elevated P. Conclusion(s) From the present meta-analysis it appears that early elevated P levels are associated with a lower OPR in GnRH antagonists. The incidence of such a condition, however, is low. Clinical Trial Registration Number NCT00866034.