The influence of microstructure on dilatation onset is analyzed in polyethylene (PE) under tension. Tests are performed by means of a video-controlled testing system that gives access to true stress sigma(33)-true strain epsilon(33) curve and records volume strain c, during stretching. The results indicate that the strain epsilon(d)(33) and the corresponding stress sigma(d)(33) from which viscoplastic dilatation begins depends on microstructural properties of PE. At microscopic scale, materials having a low epsilon(d)(33) are characterized by inhomogeneous deformation mechanisms leading to pronounced crazing phenomena in amorphous layers. On the contrary, materials having a high epsilon(d)(33) involve homogeneous deformation mechanisms that limit crazing. These observations are discussed on the basis of crystallinity and tie molecules density. A simple model predicting epsilon(d)(33) developed from these microstructural aspects. POLYM. ENG. SCI., 49:1198-1205, 2009. (C) 2009 Society of Plastics Engineers