BACKGROUND: Implant-related impingement has been reported following metal-on-metal hip resurfacing, and reactive osseous patterns associated with implant-bone impingement have been identified. The purpose of this study was to determine the prevalence and clinical implications of radiographic signs of femoral neck-acetabular cup impingement following metal-on-metal hip resurfacing. METHODS: Serial anteroposterior and lateral radiographs made five to 12.9 years postoperatively were available for ninety-one of the first 100 metal-on-metal hip resurfacing procedures (in eighty-nine patients) performed by the senior author. These radiographs were reviewed by a single independent observer, who was blinded to the clinical results. Radiographic signs of impingement were assessed and were correlated with clinical outcomes. RESULTS: Twenty hips (in eighteen patients) had at least one of two reactive osseous signs: a solitary exostosis (six hips, 7%) and an erosive "divot-type" deformity (twenty hips, 22%). Each radiographic sign occurred predominantly at the superior aspect of the femoral neck just distal to the femoral component. None of the patients with such an impingement sign reported any symptoms or discomfort during examination of the range of hip motion. These patients had a greater mean postoperative University of California Los Angeles activity score and a greater mean range of hip motion than the patients without an impingement sign. Based on the numbers available, there was no association between component size, abduction angle and anteversion angle of the socket, femoral stem-femoral shaft angle, or femoral component-femoral neck ratio and the occurrence of repetitive impingement signs on radiographs. CONCLUSIONS: The reactive osseous features identified in this study should facilitate the radiographic assessment of impingement in other patients following hip resurfacing arthroplasty. Longer-term follow-up is needed to determine whether radiographic signs of impingement are of prognostic consequence.