In radiotherapy planning systems, delineation of hollow normal tissue organs, such as the bladder, is time-consuming. Automated delineation may presuppose two assumptions: (1) the bladder resembles a spherical shell and (2) the volume of bladder tissue is preserved regardless of the volume of urine (luminal volume) inside. The purpose of the present study was to test these assumptions. 22 CT scans from 7 patients were studied retrospectively. Transverse cross-sectional areas enclosed by the outer contour (A(out)) and inner contour of the bladder (A(in)) were recorded from the images. Hence, the transverse cross-sectional area of the wall, A(wall)=A(out)-A(in), and the volume of bladder tissue at various luminal volumes, could be calculated. To quantify the method uncertainty, the same procedure was applied on three spherical plastic phantoms. The results were also compared with data from the Visible Human Project's photographs of cadaver cryo-sections. Assumption no. 1 stated above, implies that A(wall) is constant regardless of the level of intersection of the sphere. The data from cryo-sections revealed a positive correlation for A(wall) and A(out), in contradiction to assumption no. 1 (p<0.001). The corresponding association derived from the repetitive CT scans of patients was also statistically significant (p<0.001) although linear regression revealed a less steep slope. A relationship was found between the volume of bladder tissue and luminal volume, hence contradicting assumption no. 2 (p<0.001). In conclusion the cross-sectional wall areas of the bladder, measured from patient CT scans, increase slightly with luminal cross-sectional areas in contradiction to expected values derived from a simplistic spherical shell model. In addition, the volume of bladder tissue is related to the luminal volume. Our results may be of practical value when developing automated delineation tools in radiotherapy planning systems.