Polyethylene samples have been annealed under high pressures in order to transform the morphology and thereby make specimens, which on subsequent orientation by hydrostatic extrusion, produce high-modulus materials for comparatively low deformation ratios. The pre-extrusion morphology, as revealed by permanganic etching followed by transmission electron microscopy, is shown to be highly sensitive to the annealing conditions, especially in the vicinity of the orthorhombic/hexagonal phase boundary. The morphological changes have also been studied by gel permeation chromatography of nitrated samples and by differential scanning calorimetry. The moduli of samples, extruded to a standard extrusion ratio of 7, reflect differences in the morphology of the starting material. It is shown that the highest modulus is not obtained by annealing totally within the hexagonal phase but rather in the changeover region between it and the orthorhombic phase. Although the lamellar size can be dramatically increased by annealing within the hexagonal phase, this development occurs at the expense of reducing the integrity of the molecular network so that for high lamellar size the modulus falls and the samples become increasingly incoherent.