Background: Slot-scanning EOS radiography is increasingly utilised in skeletal imaging. It has been shown that EOS often reveals significant incidental findings including cardiopulmonary abnormalities. Furthermore, the slot-scanning image acquisition of EOS can substantially influence depiction of cardiac morphology, which can be confusing for those unfamiliar with the technique. Patients and methods: We aimed to explore the relationship between the depicted cardiac anatomy EOS and chest x-ray, by analysis of the differences in the measurements of the cardiothoracic ratio in 52 patients who had an erect, postero-anterior chest radiograph in deep inspiration, with less than a year difference to the EOS spine radiograph. Internal chest and cardiac diameter were measured, and the cardiothoracic ratio (CTR) was calculated as a surrogate marker for gross cardiac size. Results: We have found that EOS consistently yields a lower internal chest diameter, and as a result higher CTR (P < 0.0001). However CTR measured on EOS and plain radiograph show strong correlation (P < 0.0001). Bland-Altman plots also revealed a consistently higher CTR on EOS, and a high intra-individual variability of cardiac and chest diameters on EOS vs. x-ray. CTR measured on EOS showed correlation with age (P < 0.05). Discussion: We show that the CTR on EOS radiographs is consistently higher compared to the chest radiography, and while cardiac diameter shows only minimal difference, chest diameter measured on EOS is markedly lower. This should be considered by those reporting spine EOS imaging to avoid overcalling incidental cardiomegaly.  Kulcsszavak: EOS, slot-scanning radiography, cardiac size, chest x-ray