Abstract Background With the aim of obtaining more uniformity and quality in the treatment of corpus uteri cancer in Belgium, the EFFECT project has prospectively collected detailed information on the real-world clinical care offered to 4063 Belgian women with primary corpus uteri cancer. However, as data was collected on a voluntary basis, data may be incomplete and biased. Therefore, this study aimed to assess the completeness and potential selection bias of the EFFECT database. Methods Five databases were deterministically coupled by use of the patient’s national social security number. Participation bias was assessed by identifying characteristics associated with hospital participation in EFFECT, if any. Registration bias was assessed by identifying patient, tumor and treatment characteristics associated with patient registration by participating hospitals, if any. Uni- and multivariable logistic regression were applied. Results EFFECT covers 56% of all Belgian women diagnosed with primary corpus uteri cancer between 2012 and 2016. These women were registered by 54% of hospitals, which submitted a median of 86% of their patients. Participation of hospitals was found to be biased: low-volume and Walloon-region centers were less likely to participate. Registration of patients by participating hospitals was found to be biased: patients with a less favorable risk profile, with missing data for several clinical-pathological risk factors, that did not undergo curative surgery, and were not discussed in a multidisciplinary tumor board were less likely to be registered. Conclusions Due to its voluntary nature, the EFFECT database suffers from a selection bias, both in terms of the hospitals choosing to participate and the patients being included by participating institutions. This study, therefore, highlights the importance of assessing the selection bias that may be present in any study that voluntarily collects clinical data not otherwise routinely collected. Nevertheless, the EFFECT database covers detailed ...