Background For patients with severe endometriosis, the spontaneous pregnancy rates have been reported to be near 0 due to extreme distortion of normal pelvic anatomy.Surgery is one of the treatment options; however, if patients failed to conceive after surgery, in vitro fertilization (IVF) is effective.The objective of this retrospective study was to determine the clinical characteristics of IVF/intracytoplasmic sperm injection (ICSI) in patients with stage Ⅲ/Ⅳ endometriosis, and to determine the impact of the interval from surgery to IVF/ICSI on outcome.Methods One hundred and sixty patients who were diagnosed with stage Ⅲ/Ⅳ endometriosis underwent IVF/ICSI cycles between February 2004 and June 2009 were enrolled.The mean interval from surgery to IVF, number of oocytes retrieved, fertilization rate, implantation rate, embryos transferred, and good embryos transferred were compared between two age groups (≤35 years and ＞ 35 years).Results The mean interval from surgery to IVF was (37.9±28.9) months for the group ≤ 35 years of age and (57.6±39.7)months for the group ＞35 years of age.Twenty-five IVF/ICSI cycles (12.8%) were performed during the first year after surgery, and 34.9% IVF/ICSI cycles were performed 2 years after surgery.No significant differences existed between the two groups with respect to the fertilization rate, implantation rate, number of embryos transferred, number of good embryos,clinical pregnancy rates, live birth rates, and cumulative clinical pregnancy rates (P ＞0.05).The probability of cumulative clinical pregnancies was 75%, 50%, and 25% ((29.0±4.8), (61.0±7.6), and (120.0±16.9) months after surgery, respectively).Conclusions For infertile patients with stage Ⅲ/Ⅳ endometriosis, the optimal time to conceive by IVF/ICSI is ＜2 years after surgery; nevertheless, most of the patients took a longer time to conceive.