Background: Outcome after desmotomy of the accessory ligament of the deep digital flexor tendon (AL-DDFT) to treat flexural deformity of the distal interphalangeal joint has been reported to be excellent. However, no studies have compared long-term athletic performance of sport horses exposed to desmotomy of the AL-DDFT to that of matched controls. Objectives: The objective of this study is to compare long-term athletic performance in sports horses subjected to desmotomy of the AL-DDFT with the performance of matched controls. Study design: This is an observational multicentre retrospective matched cohort study. Methods: Records from horses undergoing desmotomy of the AL-DDFT between 2004 and 2015 were reviewed. Various databases were used to identify age-matched siblings as unexposed controls and data on the horses' athletic careers. Results: Seventy-four exposed and 194 matched unexposed horses were included. Although not significantly different, the proportion of exposed horses entering competition (28%, 95% CI 16%-38%) had a substantial risk difference compared with the proportion of unexposed horses entering competition (38%, 95% CI 26%-44%) (P =.2). Career longevity was significantly better for unexposed (15.6 competitions [95% CI 10.7-22.5] over a median of 570 days [IQR 210-1340]) than for exposed horses (9.7 competitions [95% CI 6.4-14.6] over a median of 219 days [IQR 2-1159] for horses operated in one limb and 6.1 competitions [95% CI 3.6-9.9] over a median of 446 days [IQR 23-603 days] for horses operated in two limbs, P <.001). Age at surgery and whether the condition was unilateral or bilateral did not affect chance of competing. Main limitations: Small sample size, varying quality of medical records, stage of contraction not noted in many records and missing information on reasons for not entering into competition. Conclusions: Desmotomy of the AL-DDFT is associated with decreased long-term athletic performance in sport horses compared with matched unexposed horses.