Endometriosis occurs when the endometrium grows in abnormal locations outside of the womb, resulting in pain and reduced quality of life. See-and-treat surgery removes endometriotic lesions but the risk of recurrence is high. A HTA commissioning call asked to evaluate the long-term (three years) effectiveness of post-operative long-acting reversible contraceptives (LARCs) in preventing recurrence. A survey of gynaecologists indicated there was no consensus about which LARC (LNG-IUS or DMPA) or comparator (COCP or no treatment) should be evaluated. We designed a ‘flexible-entry’ internal pilot to assess whether a four-arm trial was feasible in light of possible strong patient preferences.