To examine the effect on pain and quality of life for women with all stages of endometriosis undergoing laparoscopic surgery compared with placebo surgery. A randomized, blinded, crossover study. A tertiary referral unit in a district general hospital. Thirty-nine women with histologically proven endometriosis completed the 12-month study. Women were randomized to receive initially either a diagnostic procedure (the delayed surgical group) or full excisional surgery (the immediate surgery group). After 6 months, repeat laparoscopy was performed, with removal of any pathology present. The end points were changes from baseline values of visual analogue pain scores, validated quality-of-life instruments (EQ-5D and SF-12), and sexual activity questionnaire scores. Patients and assessors of outcomes were blinded to the treatment-group assignment. Significantly more of the 39 women operated on according to protocol reported symptomatic improvement after excisional surgery than after placebo: 16 of 20 (80%) vs. 6 of 19 (32%); χ 2(1) = 9.3.Other aspects of quality of life were also significantly improved 6 months after excisional surgery but not after placebo. Progression of disease at second surgery was demonstrated for women having only an initial diagnostic procedure in 45% of cases, with disease remaining static in 33% and improving in 22% of cases. Nonresponsiveness to surgery was reported in 20% of cases. Laparoscopic excision of endometriosis is more effective than placebo at reducing pain and improving quality of life. Surgery is associated with a 30% placebo response rate that is not dependent on severity of disease. Approximately 20% of women do not report an improvement after surgery for endometriosis.