Replacement of a natural hip joint with an artificial is one of the most commonly used surgical procedures in orthopaedic surgery. This paper describes the methodology of the hip joint prosthesis design, which presents the foundation for the automation of all stages of the development of custom-made prostheses. A general mathematical model based mostly on B-spline surfaces is used in order to define the prosthesis geometry. In order to verify the method, a software solution was developed and used for designing prostheses based on ten diagnostic images obtained by various methods. The results of the analysis of the ten designed prostheses, out of which five manufactured, show that deviations from the desired values are less than the errors caused by diagnostic imaging.