OBJECTIVE: To evaluate the impact of endometriosis on IVF-ET cycles and to compare IVF outcomes between stage I/II and stage III/IV endometriosis. METHODS: We analyzed 697 patients (1,199 cycles) with endometriosis (stage I-II: 638 cycles, stage III-IV: 561 cycles) and 325 pts (459 cycles) with tubal factor as controls between January 1994 and April 2004. Pts with endometriosis were diagnosed by laparoscopy and medical and surgical treatment were done in 353 cycles (55.3%) and 466 cycles (83.1%) of stage I-II/stage III-IV endometriosis. Cycles with age>35 years or FSH>20 mIU/mL or severe male factor infertility were excluded. RESULTS: The number of retrieved oocytes (9.97+/-7.2 vs. 13.4+/-7.9 (p<0.0001)), total number of embryos (6.5+/-4.8 vs. 9.1+/-5.6 (p<0.0001)), and good quality embryos (2.43+/-1.6 vs. 2.74+/-1.7 (p=0.013)) significantly decreased in stage III-IV endometriosis than in control. But pregnancy rate of stage III-IV endometriosis was comparable with control (35.7% vs. 36.8%). Fertilization rate and number of total embryos were lower in stage I-II endometriosis than in control (64.8+/-22.9 vs. 70.8+/-20.8 (p<0.0001), 7.6+/-5.0 vs. 9.1+/-5.6 (p<0.0001)). In patients with medical and surgical treatment of endometriosis, pregnancy rate and live birth rate was significantly lower in stage I-II than in stage III-IV endometriosis (29.2 vs. 36.2 (%), p=0.045, 23.9 vs. 31.5 (%), p=0.043). There was no difference in the mean age, but the duration of infertility was significantly longer (56.5+/-26.3 vs. 46.9+/-25.8 (mon), p<0.0001) and fertilization rate was lower (64.7+/-23.3 vs. 70.5+/-22.7 (%), p=0.001) in stage I-II than stage III-IV endometriosis. CONCLUSION: We suggest that IVF should be considered earlier in patients with minimal to mild endometriosis because of significantly decreased fertilization rates.