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Background . In daily practice, assisted reproductive technology (ART) cycles are often cancelled under the assumption that a prolonged stimulation period lowers the likelihood of an appropriate ovarian response. The aim of the present study was to determine whether a prolonged cycle has an adverse effect on achievement of pregnancy. Methods . The study sample included consecutive women enrolled in our ART unit between 1999 and 2001 who were treated with the mid-luteal long suppressive gonadotropin-releasing hormone protocol. Data were collected prospectively on a computerized database and evaluated at the end of the study. Prolonged stimulation was defined as a stimulation period of more than two standard deviations (SD) above the mean. Outcome was compared between patients who required prolonged stimulation and those who did not. Results . A total of 1015 consecutive in vitro fertilization (IVF) cycles were performed with the mid-luteal long suppressive protocol during the study period. Thirty-four women required prolonged stimulation. No difference in clinical pregnancy rate was detected between women who received prolonged stimulation and those who did not (9/34, 26.5% vs. 291/981, 29.7%), despite the significantly fewer oocytes retrieved in the prolonged-stimulation group (7.1 ± 5.2 and 11.6 ± 6.7 (mean ± SD), respectively, p < 0.001).Conclusion . The likelihood of achieving pregnancy is not influenced by the length of stimulation. We recommend that IVF cycles should not be discontinued on the grounds of prolonged stimulation alone. [ABSTRACT FROM AUTHOR]