Objective To evaluate the effect of self-administered isoflurane and desflurane on women's experience of outpatient treatment at colposcopy. Design A prospective double-blinded randomised controlled trial.Setting A colposcopy clinic serving a regional population.Population Three hundred and ninety-six women scheduled for treatment of cervical intraepithelial neoplasia (CIN) by large loop excision of the transformation zone (LLETZ). Methods Self-administration of trial gas during a LLETZ procedure. One hundred and ninety-eight women were randomised to use isoflurane and desflurane and 198 to use placebo. Main outcome measures Patient satisfaction, pain and anxiety.