['To analyze the potential association between serum cetrorelix levels and clinical pregnancy outcome in patients who had undergone assisted reproduction cycles with a GnRH antagonist cetrorelix acetate 3-mg injection.', 'Retrospective case-control study.', 'University-affiliated private-assisted reproduction center.', '130 IVF and intracytoplasmic sperm injection first cycles, treated with the same cetrorelix acetate protocol, in two matched groups according to whether the cycle resulted in clinical pregnancy (n = 56) or not (n = 74).', 'Cetrorelix acetate administration at 3 mg in a sandwich protocol.', 'Serum cetrorelix concentrations on the day of hCG administration with regard to clinical pregnancy outcome, pre- versus post-hCG percent change in serum E(2) levels and implantation rates.', 'The cetrorelix serum concentrations were in the range of 0.29 to 5.12 ng/mL. The comparisons between groups with and without clinical pregnancy revealed comparable serum cetrorelix levels. There was no significant correlation between the serum cetrorelix concentrations and percent change in pre- versus post-hCG serum E(2) levels. Serum cetrorelix levels were comparable among patients with various implantation rates.'] Do serum cetrorelix concentrations affect clinical pregnancy outcome in assisted reproduction?