This clinical trial was conducted in 2012 in several healthcare facilities of Russia, including at the Department of Gynecology, Moscow Regional Research Institute of Obstetrics and Gynecology, Ministry of Health of the Moscow Region.Objective: to investigate the efficacy and safety of Buserelin-long FS at a dose of 3.75 mg, a lyophilisate (JSC “F-Sintez”, Russia) to prepare a long-acting intramuscular suspension in late reproductive-aged patients with uterine endometriosis. Subjects and methods. The clinical trial protocol included 50 patients aged 35 to 45 years who were diagnosed with echographically and hysteroscopically verified and unverified Stages I, II, and III uterine endometriosis and treated with Buserelin-long FS; its therapeutic efficacy was further evaluated. Results. The therapeutic efficacy of Buserelin-long FS in 44 patients was ascertained to meet all the established four criteria for the combined endpoint of the trial: a reduction in the values of luteinizing hormone less than 7.4±7.3 mIU/ml, follicle-stimulating hormone less than 7.0±5.6 mIU/ML, estradiol less than 110.0 pmol/l, and median uterine echo thickness <4—6 mm. Six patients showed an inconsistency in 1 criterion: a decrease in estradiol levels in 5 patients and a magnitude of the reduction in the median uterine echo in 1. There were no health problems or early termination of the trial because of adverse or serious undesirable events. The long-term results of the trial (within 3 years after its completion) indicated that 86% of the patients had better quality of life after the therapy. Four patients gave birth to healthy babies. Conclusion. The use of this agent is a safe and effective treatment for uterine endometriosis, which ensures the highest clinical effect with minimal adverse reactions, making it the drug of choice in treating this disease. The authors declare no conflicts of interest.