Introduction Deep myometrial invasion (>= 50%) is a prognostic factor for lymph node metastases and decreased survival in endometrial cancer. There is no consensus regarding which pre/intraoperative diagnostic method should be preferred. Our aim was to explore the pattern of diagnostic methods for myometrial invasion assessment in Sweden and to evaluate differences among magnetic resonance imaging (MRI), transvaginal sonography, frozen section, and gross examination in clinical practice. Material and methods This is a nationwide historical cohort study; women with endometrial cancer with data on assessment of myometrial invasion and FIGO stage I-III registered in the Swedish Quality Registry for Gynecologic Cancer (SQRGC) between 2017 and 2019 were eligible. Data on age, histology, FIGO stage, method, and results of myometrial invasion assessment, pathology results, and hospital level were collected from the SQRGC. The final assessment by the pathologist was considered the reference standard. Results In the study population of 1401 women, 32% (n = 448) had myometrial invasion of 50% of more. The methods reported for myometrial invasion assessment were transvaginal sonography in 59%, MRI in 28%, gross examination in 8% and frozen section in 5% of cases. Only minor differences were found for age and FIGO stage when comparing methods applied for myometrial invasion assessment. The sensitivity, specificity, and accuracy to find myometrial invasion of 50% or more with transvaginal sonography were 65.6%, 80.3%, and 75.8%, for MRI they were 76.9%, 71.9%, and 73.8%, for gross examination they were 71.9%, 93.6%, and 87.3%, and for frozen section they were 90.0%, 92.7%, and 92.0%, respectively. Conclusions In Sweden, the assessment of deep myometrial invasion is most often performed with transvaginal sonography, but the sensitivity is lower than for the other diagnostic methods. In clinical practice, the accuracy is moderate for transvaginal sonography and MRI.