Abstract: Objective-To characterize the clinical features, diagnostic procedures, treatment, and outcome for horses with an exostosis on the palmar or plantar cortex of the third metacarpal bone (MC3) or third metatarsal bone (MT3). Design-Retrospective case series. Animals-16 horses. Procedures-Records from 2001 through 2010 were evaluated to identify horses with radiographic and ultrasonographic evidence of an exostosis on an MC3 or MT3. Signalment, history, lameness examination results, diagnostic imaging results, surgical and histopathologic findings, treatment, and outcome were evaluated. Results-9 horses (group A) had unilateral lameness of the exostosis-affected limb that was alleviated with local or perineural analgesia. Seven horses (group B) had inconsistent lameness of the affected limb. The exostosis was located between the middle and distal third of the MC3 or MT3 in all horses. Desmopathy or peritendinous fibrosis (or both) of the suspensory ligament at the exostosis site was identified in 6 horses. All horses in group A returned to full function after conservative or surgical management. Lameness did not recur in any of the horses in group B despite no treatment or only conservative management. Conclusions and Clinical Relevance-Exostosis of the palmar cortex of an MC3 or plantar cortex of an MT3 should be considered as a cause of lameness in horses. The diagnosis can be made by the use of radiography and ultrasonography combined with specific diagnostic analgesia. Prognosis for return to athletic activity can be good but should be modified contingent on concurrent desmopathy of the suspensory ligament.