Background: Endometriosis is a common and disabling gynecologic condition affecting between 5% and 15% of women of childbearing age. Conventional medical intervention has unpleasant side-effects, and symptoms frequently return after treatment. Preliminary evidence suggests Chinese herbal medicine (CHM) may contribute to the treatment of endometriosis. Objectives: The aims of this study were to test the feasibility of a novel methodology for investigating individualized decoctions of CHM rigorously and to gather preliminary data on the treatment effect of CHM for a larger definitive trial. Design: This was a 16-week prospective, double blinded, randomized controlled trial of 40 women with laparoscopically confirmed endometriosis. Settings: The trial was conducted at a private CHM clinic in Hove (U.K.) and at a National Health Service outpatient clinic in London (U.K.). Interventions: Participants were initially randomized to either wait-list control (WLC) or treatment groups to receive either individualized CHM decoctions or a therapeutically inert placebo decoction. Outcome measures: Four 10-cm visual analogue scales (VAS) were used to measure menstrual pain, daily pain, and pain on intercourse and bowel movement; these measurements were recorded weekly. The Endometriosis Health Profile-30 (EHP-30) endometriosis-specific quality-of-life questionnaire was completed at the beginning and at the end of the trial. The Measure Yourself Medical Outcomes Profile (MYMOP) a patient-centered health questionnaire was completed monthly. Liver and renal function was measured at 0, 4, 8, and 16 weeks. Results: Twenty-eight (28) women completed the trial. High dropout rates led to the suspension of the WLC. Randomization, double blinding, and allocation concealment was achieved successfully. Adjusted mean differences favored the active treatment in the EHP-30 and MYMOP scores. VAS scores favored the active treatment for relief of menstrual pain and the placebo group for reduction of daily pain. Conclusions: the methodology successfully allowed individualized CHM decoctions to be tested rigorously. There are nonspecific contextual effects from CHM that require further investigation. Provisional data were generated to warrant a larger, more-definitive study.