The aim of this study was to evaluate the ultrasonographic changes taking place in the uterus after thermal balloon endometrial ablation therapy. The ultrasonographic findings in the uterus after thermal balloon endometrial ablation in 13 patients were compared to those found in 13 patients treated with gestagen therapy. Examinations took place before initiation of the treatment, on the 1st postoperative day (thermal balloon ablation group only), and 1 and 6 months later. One month after thermal balloon endometrial ablation therapy, the ultrasonographic examination revealed a clear-limited hyperechogenic zone surrounding the uterine cavity which was filled with fluid in 6 out of 10 patients. The ultrasonographic findings may represent a burn-injury-induced obstruction in the cervical canal with subsequent collection of fluid in the uterine cavity and a zone of coagulation in the submucosal layers. At the final examination in the thermal balloon group, the mean endometrial thickness was lower than the pretreatment level. No changes were observed in the gestagen therapy group.