Synchrotron X-ray microtomography at submicrometer resolution was used for investigating the evolution with tensile deformation (up to true strains of 1.8) of the microstructure of a semi-crystalline polymer. Different types of data treatment were performed on the obtained image stacks. Combining analysis in real space with 2D FFT transforms, we were able to show that deformed high density polyethylene does not contain any cavities larger than 1 mu m, which contradicts the commonly admitted assumption that whitening of El DIE upon necking is associated with a crazing mechanism. In contrast, X-ray tomography reveals that HDPE deformation leads to the development of density contrasts, the spatial repartition of which is strongly anisotropic.