The possible relationship between endogenous LH concentrations and clinical outcome was evaluated in 750 patients treated with a standardized gonadotrophin-releasing hormone (GnRH) antagonist and recombinant FSH (rFSH)-only protocol. Serum LH concentrations were measured during stimulation by a central laboratory and patients were stratified into quantiles of <P25, P25-P75, and >P75. The P25 values were 3.38 IU/l, 0.93 IU/l, and 0.91 IU/l on stimulation days 1, 5, and 8, respectively. The ongoing pregnancy rates per started cycle of patients within the <P25 subset on stimulation day 1, 5, or 8 were highly similar to those in the P25-P75, and >P75 subsets and ranged in the various subsets between 35.0% and 39.5%. In keeping with previous, smaller studies, these findings demonstrate that in good prognosis, non-obese patients endogenous LH in a GnRH antagonist protocol is able to support treatment with rFSH only.