Objective - To report a severe form of carpometacarpal osteoarthritis (CMC-OA) affecting primarily older Arabian horses. Study Design - Retrospective study. Animals - Thirty-one horses with CMC-OA. Methods - Carpal radiographs (405 sets) from 3 hospitals were evaluated to identify horses with marked osteoproliferative reaction across the medial aspect of the CMC joint. Owners were contacted to obtain detailed histories and status updates. Necropsy specimens of the CMC joint were evaluated at 2 hospitals to determine the prevalence of 2 variations in the articulation between the proximal second and third metacarpal bones. Results - Thirty-one horses were identified as having marked osteoproliferative reaction at the CMC joint. Twenty-three (74%) were Arabian horses. Of the Arabian horses, the average age at admission was 14.4 years. Eight (34.8%) Arabian horses had a known history of trauma. Most were no longer rideable at presentation. Ten of the horses were subsequently euthanatized because of lameness. The dorsal and palmar articulations between the second and third metacarpal bones were examined in 177 horses. The palmar articulation was absent in 48% of Arabian horses and 12.5% of non-Arabian horses at 1 center, including 4 horses with CMC-OA. At the second center, the palmar articulation was present in 8 of 8 Arabian horses but was absent in 22 of 92 (24%) non-Arabian horses. Conclusions - An increased frequency of this crippling form of OA was observed in Arabian horses. It may reflect an increased prevalence, in some geographical regions, of an absent palmar articulation between the second and third metacarpal bones. Clinical Relevance - Carpal trauma in some Arabian horses may result in unexpectedly severe carpometacarpal osteoarthritis.