BACKGROUND: Metal-on-metal arthroplasty-related pseudotumors can cause severe local destruction of bone and soft tissues. The cause of pseudotumors is unknown, although some authors have implicated metal wear debris. The aim of this study was to measure the location and magnitude of wear on resurfacing devices that were retrieved during revision procedures for pseudotumor (the pseudotumor group) and for other reasons (the control group). METHODS: We examined thirty-six hip-resurfacing implants, which were divided into two groups: eighteen implants from patients with a diagnosis of pseudotumor and eighteen control implants. Implant orientation and patient demographics were recorded. Three-dimensional, contactless metrology was used to scan the surface of the femoral and acetabular components to a resolution of 20 nm. Linear and volumetric wear were measured, and the components were examined for evidence of edge wear. RESULTS: There was three times more total linear wear and over six times more total volumetric wear of the femoral and acetabular components in the pseudotumor group as compared with that in the control group. The mean linear wear rate and standard deviation of the femoral components in the pseudotumor group (8.4 ± 8.7 μm/yr) were significantly greater than those in the control group (2.9 ± 3.9 μm/yr; p = 0.01). The mean volumetric wear rate of the femoral components was also significantly greater in the pseudotumor group (3.3 ± 5.7 mm3/yr) than it was in the control group (0.8 ± 1.2 mm3/yr; p = 0.009). Seventeen of eighteen subjects in the pseudotumor group had edge wear, compared with six of eighteen in the control group (p