['The aim of the present study was to evaluate the association between clinical pregnancy and serum luteinizing hormone (LH) levels, assessed after 14 days of endometrial preparation with estradiol (E(2)) in the absence of pituitary suppression during a frozen-thawed embryo transfer (FRET) cycle.', "A total of 513 patients undergoing their first FRET cycle (01/99 to 11/05) participated in this prospective study. Endometrium preparation for FRET was started on cycle day 1 and continued for a fixed period of 14 days with trans-dermal E(2) patches. On day 14, serum LH, progesterone and E(2) levels were assessed. On day 15, progesterone supplementation was initiated and patients underwent embryo transfer on day 17 or day 18. The association between clinical pregnancy and LH levels was evaluated in groups of patients defined according to Tukey's Hinges percentile analysis of LH levels on day 14. In addition, robust logistic regression was performed with the dependent variable clinical pregnancy and independent variables LH, progesterone, embryos score, cycle rank and gravidity.", 'Age, BMI, parity, cycle rank, embryo number, embryo score, endometrial diameter, E(2) and progesterone were not significantly different in cycles with low (0.1-8.1 IU/l; n = 132), intermediate (8.2-19.4 IU/l; n = 238) and high (20.0-78.0 IU/l; n = 143) levels of LH, respectively. Clinical pregnancy rates were not significantly different in cycles with low [12.1%, 95% confidence intervel (CI) 7.6-18.8], intermediate (13.4%, 9.7-18.4) and high levels of LH (16.1%, 11.0-23.0). Robust logistic regression analysis indicated that embryo score [Odds ratios (OR) 1.04, 95% CI 1.02-1.06, P < 0.01] was statistically significantly associated with the likelihood of clinical pregnancy achievement, but not day 14 levels of LH or progesterone, gravidity or cycle rank.'] Are mid-cycle serum levels of endogenous LH associated with the likelihood of pregnancy in artificial frozen-thawed embryo transfer cycles without pituitary suppression?