OBJECTIVE:To develop a magnetic resonance imaging (MRI) atlas for the assessment of osteoarthritis (OA) of the first metatarsophalangeal joint (1st MTPJ), and to assess its intra- and inter-examiner reproducibility. METHODS:MRI (proton density with and without fat suppression) was performed on the 1st MTPJ of 60 participants (30 with and 30 without 1st MTPJ OA). Characteristic MRI features of OA were then used to develop an MRI atlas of 1st MTPJ OA. The atlas assessed osteophytes (dorsal metatarsal head, plantar metatarsal head, dorsal proximal phalanx), bone marrow lesions (metatarsal head, proximal phalanx, sesamoids), cysts (metatarsal head, proximal phalanx), effusion-synovitis (dorsal, plantar), joint space narrowing (metatarsal-proximal phalanx, metatarsal-sesamoids) and cartilage loss. To assess the reproducibility of the atlas, two examiners independently rated the MRIs of 30 participants on two occasions. Intra- and inter-examiner reproducibility were determined using percentage agreement and Gwet's AC1. RESULTS:Observations using the atlas demonstrated fair to perfect intra-examiner reproducibility (percentage agreement from 67 to 100% and Gwet's AC1 from 0.38 to 1.00), and fair to almost perfect inter-examiner reproducibility (percentage agreement from 67 to 98% and Gwet's AC1 from 0.40 to 0.96). CONCLUSION:A MRI scoring system for the assessment of osteoarthritis of the 1st MTPJ has been developed. The atlas demonstrates excellent intra- and inter-examiner reproducibility. The atlas has the potential to allow for a better understanding of the cause(s) of pain in 1st MTPJ OA. This article is protected by copyright. All rights reserved.