The effect of the combination of pressure annealing and subsequent hydrostatic extrusion on some mechanical properties of composites of hydroxyapatite in a polyethylene matrix is examined. Both linear and branched polyethylenes have been used as the matrix and it is found that composites with the former can be processed to give the higher modulus. An important practical finding is that products with a Young's modulus well into the range shown by cortical bone can be produced. The critical step in the enhancement of the modulus is pressure annealing which alters the morphology of the linear polyethylene, encouraging the development of both crystallinity and crystallite size. The presence of butyl branches along the molecular backbone limits the extent to which these can be developed by pressure annealing and accounts for the failure of the process to improve the modulus of composites using the branched material. Comparison with similarly prepared samples of the pure polyethylenes shows that the development of orientation in the polyethylene is considerably restricted by the presence of hydroxyapatite particles, irrespective of whether pressure annealing is performed prior to extrusion. Consequently, the properties of these composites are less than might be expected from studies on the isolated polyethylenes.