This paper presents the methodology for user-customized design of a class of one-of-a-kind assistive devices. This class consists of passive, articulated mechanical manipulation aids, which are physically coupled to the user and therefore, must be customized to the user. Geometric and kinematic measurements of the user are used to create a virtual model of the user. The design of the customized product is based on kinematic synthesis and simulation. An integrated virtual environment, with a virtual model of the user interacting with the product, allows the testing, iterative re-design, and evaluation of the product. Geometric and kinematic data acquisition, mechanism design and analysis, CAD/CAM and visualization modules aid the designer in this process. A head-controlled feeding aid for quadriplegics is used to illustrate the approach.