Background We sought to analyze the toxicity rates and the treatment outcomes in endometrial cancer (EC) patients treated with postoperative three-dimensional conformal radiotherapy (3DCRT) and intensity-modulated radiotherapy (IMRT). Material and methods The clinical data of 646 EC patients treated with postoperative adjuvant 3DCRT (265 patients, 41%) or with IMRT (381 patients, 59%) between April 2007 and August 2019 were retrospectively analyzed. The primary endpoints were treatment-related acute and late gastrointestinal (GI) and genitourinary (GU) toxicities. The secondary endpoints were LC and overall survival (OS) and disease-free survival (DFS). Results Median follow-up time was 37 months. The rates for acute GI and GU toxicities of any grade for the entire group were 55.6% and 46.8%, respectively. Acute grade >= 2 GI toxicity was significantly less in patients treated with IMRT compared to those treated with 3DCRT (11.0% vs. 19.2%, p=.004). However, no significant difference grade >= 2 GU toxicities was observed between the 3DCRT and IMRT groups (15.1% vs. 11.0%; p=.15). Acute grade >= 2 GI and GU toxicities were higher in patients receiving systemic chemotherapy, while paraaortic field irradiation increases only the risk of acute grade >= 2 GI toxicity. Estimated 3-year late grade >= 3 GI toxicity rates in the 3DCRT- and IMRT-treated patients were 4.6% and 1.9% (p= .03), respectively. The patients treated with adjuvant ChT had higher rates of late serious GI complications than those without adjuvant ChT. No significant difference in terms of survival and disease control was observed between the 3DCRT and IMRT treatment groups. No significant factor for LC was found in the multivariate analysis. Conclusion In this multicentric study involving one of largest patient population, we found that IMRT-treated EC patients showed comparable clinical outcomes but with a lower incidence of GI toxicities compared with those treated with 3DCRT.