ObjectivesTo investigate whether bone erosions in patients with rheumatoid arthritis (RA) show evidence of repair. Methods 127 erosions were identifi ed in metacarpophalangeal joints 2-4 of the right hands of 30 RA patients treated with tumour necrosis factor inhibitors (TNFi) and 21 sex, age and disease activitymatched patients treated with methotrexate. All erosions were assessed for their exact maximal width and depth by high-resolution µCT imaging at baseline and after 1 year. Results All erosions detected at baseline could be visualised at follow-up after 1 year. At baseline, the mean width of bone erosions in the TNFi group was 2.0 mm; their mean depth was 2.3 mm, which was not signifi cantly different from the methotrexate-treated group (width 2.4 mm; depth 2.4 mm). Mean depth of erosions signifi cantly decreased after 1 year of treatment with TNFi (−0.1 mm; p=0.016), whereas their width remained unchanged. In contrast, mean depth and width of erosive lesions increased in the methotrexate-treated group. The reduction in the depth of lesions was confi ned to erosions showing evidence of sclerosis at the base of the lesion. Moreover, deeper lesions in the TNFi group were particularly prone to repair (−0.4 mm; p=0.02) compared with more shallow lesions. Conclusions Bone erosions in RA patients treated with TNFi show evidence of limited repair in contrast to bone erosions in patients treated with methotrexate. Repair is associated with a decrease in the depth of lesions and sclerosis at the bases of the lesions. Repair thus emerges from the endosteal rather than periosteal bone compartment and probably involves the bone marrow.Bone erosion is a central pathophysiological process in rheumatoid arthritis (RA). Growth factors such as monocyte colony stimulating factor as well as cytokines such as receptor activator of nuclear factor kappa B ligand, tumour necrosis factor alpha (TNFα) and interleukin-1 expressed in the synovial tissue stimulate the differentiation of monocytes/ macrophages into osteoclasts, which initiate the resorption of the periarticular bone. 1 2 This resorption process leads to a loss of mineralised tissue along the joints, which can be visualised as bone erosion by conventional radiography. Apart from clinical measurements, the detection and quantifi cation of bone erosion is an important outcome parameter in both clinical studies and clinical practice. 3 4 Virtually all currently used drug therapies have been tested for their ability to retard the bone erosive process in RA, which is suggestive of effective disease control and preservation of the joint architecture.Bone erosion is closely linked to infl ammatory disease activity of RA. The better synovial infl ammation can be controlled by anti-infl ammatory drug therapy, the more likely retardation or arrest of bone erosion is achieved. Despite this link between infl ammation and bone erosions, some treatment regimens are particularly effective to retard structural damage. 5 Inhibition of tumour necrosis factor alpha inhibibors (...