['To determine if arthroscopic surgery performed on weanlings/yearlings with trochlear ridge OCD of the femur (stifle OCD), or intermediate ridge of the tibia (tarsal OCD) detected on routine presale radiographs affects future racing performance.', 'Retrospective case control study.', 'Thoroughbred weanlings and yearlings.', 'Stifle (N = 37; 22 male, 15 female) and tarsal (N = 35; 22 male, 13 female) OCD cases were identified. Outcome measures relating to racing performance (number of race starts, wins, places, and prize money earned, from races in their 2- and 3-year-old racing careers and in total from the period studied) were defined and then cases were compared to 2 age and sex matched controls using multilevel linear regression models. The effect of horse age at surgery and surgeon experience on the outcomes for cases with stifle OCD were also examined.', 'Stifle OCD cases (N = 37; age 190-563 days at surgery) had significantly lower total earnings (P = .043), fewer total starts (P = .001) and fewer total 1st place finishes (P = .003) than their matched controls. For tarsal OCD cases (N = 35, age 127-470 days at surgery), fewer starts made by cases than controls (P = .018). Younger horse age at time of surgery and reduced surgeon experience were significantly associated with worse outcomes for stifle OCD cases.'] Do treatment of osteochondrosis dissecans in the stifle and tarsus of juvenile thoroughbred horses?