Endometrial cancer is the most frequently diagnosed gynecological tumour. Transvaginal ultrasound has a leading role in the preoperative evaluation of endometrial cancer patients. The study aimed to identify factors that can worsen the diagnostic accuracy of transvaginal ultrasound in endometrial cancer patients. We retrospectively analysed 290 patients with histological diagnosis of endometrial adenocarcinoma. Two-dimensional (2D) gray-scale ultrasound and power Doppler imaging were performed. Age, menopause status, obesity, parity, Figo stage and benign uterine disorders were evaluated as possible factors worsening the diagnostic accuracy of the ultrasonography. FIGO stage IB was the main significant confounding factor in the univariate analysis (p = .004). Furthermore, 2D transvaginal ultrasound showed worse diagnostic accuracy in endometrial cancer patients with concomitant benign uterine pathologies. Impact statement What is already known on this subject? Many studies have analysed the reliability and diagnostic accuracy of transvaginal ultrasound in predicting myometrial invasion, but few studies have underlined the importance of confounding factors. Shin et al. (2011) showed that diffuse fibromatosis is a quality ultrasound confounding factor. Furthermore, Fischerova et al. (2014) showed that body mass index (BMI) did not influence the diagnostic accuracy of ultrasound assessment. What do the results of this study add? FIGO stage IB is the main factor worsening the diagnostic accuracy of transvaginal ultrasound in endometrial cancer patients (p = .004). Among the 82 patients with histologically proven FIGO stage IB, 27 (32.9%) had a wrong ultrasound prediction of myometrial infiltration. Twenty-one (36.2%) patients in whom there was no agreement between ultrasound prediction of myometrial infiltration and pathological analysis had fibromatosis and/or adenomyosis (p = 0.04). What are the implications of these findings for clinical practice and/or further research? Two-dimensional ultrasound represents a useful tool in the correct pre-operative setting of patients with endometrial cancer. In FIGO stages IB endometrial cancer patients and in conjunction with benign uterine pathologies, 2D transvaginal ultrasound has less diagnostic accuracy. In these cases, MRI still plays a leading role.