Most authors agree that "fit" and "fill" of the endosteal canal is critical to the success of cementless femoral components. Yet, there exists a wide variation to the anatomic size and shape of this canal in the normal population. This paper reviews the rationale and the efficacy of using a computed tomography-generated CAD/CAM custom femoral component in cementless total hip arthroplasty. One hundred fifty-six cases (81 primary and 75 revisions) were reviewed with follow-up time of six weeks to three years (mean, 22 months). A subset of 48 hips (25 primary and 23 revisions) has been followed a minimum of two years. For the primary hips, the custom group was found to have statistically higher Harris pain scores (less pain) at all follow-up intervals as compared to a prior series by the same surgeon using an off-the-shelf (OTS) prosthesis. Revision customized hip implants had lower Harris pain and total scores than primary custom hips, but 80% were in the none or slight pain category. In revision cases, the use of custom components decreased the need for structural bone grafting and achieved stability on host bone in situations in which it was not possible using OTS components. Complications included failure by aseptic loosening of one primary and one revision case. Initial subsidence of more than 3 mm of the collarless custom design occurred in 8%, the majority being in revision cases. All cases but one appear to have stabilized. Subsidence occurred mainly in cases done early in the series.(ABSTRACT TRUNCATED AT 250 WORDS)