An investigation was performed to study the mechanical performance of fiber-reinforced composite hip prostheses. In Part I of the study, a three-dimensional finite element code was developed for analyzing a composite hip prosthesis in a femur. The material properties of the composite were treated as anisotropic and inhomogeneous while the properties of the femoral bone were treated as anisotropic and homogeneous. All the materials were assumed to behave linear-elastically. Thermoplastic graphite/PEEK material was selected for the study. No slippage was assumed at the interface between the implant and the surrounding femoral bone. In Part Il, numerical simulations were performed using the code to study the performance of a composite prosthesis in the femur. The stress/strain distributions, micromotions, and strain energy density of the surrounding femoral bone were evaluated and found to be related to initial fixation and long-term stability of the prosthesis in the femur. Numerous fiber orientations were studied, and the results of the calculations were compared with those generated from a prosthesis made of cobalt chrome and Ti-6Al-4V titanium alloys. Based on the analysis, it was shown that compared to conventional metallic implants more favorable stresses and deformations could be generated in the femur using composite implants. In addition, by changing fiber orientations according to femoral loads, a composite implant could be designed specifically for the left or the right femur. (C) 1998 John Wiley & Sons, Inc.