Study Question: What are the complications of transvaginal ethanol sclerotherapy for the treatment of endometriomas?, Summary Answer: Sclerotherapy is a reliable, minimally invasive method applicable in outpatient procedures but with specific and potential life-threatening complications that need to be identified and prevented., What Is Known Already: There are currently few data on the use of transvaginal ethanol sclerotherapy, and we mainly note septic complications., Study Design, Size, Duration: A retrospective observational cohort study was carried out. The study was conducted at an academic hospital and included 126 women aged 31.9 ± 5.5 years (mean ± SD), between November 2013 and June 2021. We analyzed a total of 157 ethanol sclerotherapy treatment (EST), treated by 131 EST procedures, in 126 women., Participants/materials, Setting, Methods: The study included women with an indication for transvaginal ethanol sclerotherapy. Indications were women with at least one endometrioma over 10 mm, isolated or associated with other endometriosis locations, requiring treatment for pain or infertility before assisted reproductive treatment. We followed a standardized transvaginal ethanol sclerotherapy procedure consisting of an ultrasound-guided transvaginal puncture of one or more endometriomas under general anesthesia. The cyst content was completely removed and flushed with saline solution. Ethanol (96%) was injected at 60% of the initial volume of the endometrioma, remained in the cyst for 10 min and was then completely removed. Ethanol loss was defined as a loss of 5 ml or more than 10% of the initial volume of the injected ethanol. Failure was defined by the contraindication of endometrioma puncture because of interposition of the digestive tract, ethanol loss in the previous endometrioma treated (in case of multiple ESTs), failure to aspirate the endometriotic fluid, contraindication to start ethanol injection owing to saline solution leakage, or contraindication to continue ethanol injection owing to suspicions of ethanol leakage at sonography. Intraoperative complications were defined by ethanol loss, positive blood alcohol level, and ethanol intoxication. Postoperative complications were defined by fever, biological inflammatory syndrome, and ovarian abscess. Complications were classified according to the Clavien and Dindo surgical classification, which is a system for classifying postoperative complications in five grades of increasing severity., Main Results and the Role of Chance: We reported a total of 17/157 (10.8%) transvaginal ethanol sclerotherapy failures during 14/131 (10.7%) transvaginal ethanol sclerotherapy procedures in 13/126 (10.3%) women. In the same sets of data, complication was reported for 15/157 (9.5%) transvaginal ethanol sclerotherapy in 13/131 (9.9%) transvaginal ethanol sclerotherapy procedures in 13/126 (10.3%) women. Nine of 126 women (7.1%) had a grade I complication, one (0.8%) had a grade II complication (medical treatment for suspicion of pelvic infection), two (1.6%) had a grade III complication (ovarian abscess) and one (0.8%) had a grade IV complication (ethanol intoxication). We did not observe any grade V complications., Limitations, Reasons for Caution: This was a retrospective study and pain assessment not considered. The benefit-risk balance of endometrioma transvaginal ethanol sclerotherapy was not evaluated., Wider Implications of the Findings: Our study is the first to evaluate the complications of transvaginal ethanol sclerotherapy with such a large cohort of women in a standardized protocol. Transvaginal ethanol sclerotherapy seems to be an effective alternative to laparoscopic surgery in the management of endometriomas and limits the alteration of ovarian reserve. Transvaginal ethanol sclerotherapy is a reliable, minimally invasive method applicable on an outpatient basis. The majority of complications are Clavien-Dindo ≤IV, for which preventative measures, or at least early diagnosis and treatment, can be easily performed. The risk of ethanol intoxication is rare, but it is a life-threatening risk that must be avoided by appropriate implementation and promotion of the sclerotherapy procedures., Study Funding/competing Interest(s): None., Trial Registration Number: Aix Marseille University's ethics committee registration number 2021-06-03-01., (© The Author(s) 2024. Published by Oxford University Press on behalf of European Society of Human Reproduction and Embryology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)