Background Shallow lucencies less than 4 mm deep into the medial femoral condyle (MFC) are frequent in Thoroughbred horses undergoing screening sales radiographs. It is unclear if these shallow defects are precursors to larger cystic lesions or if they are fully developed defects that remodel into a flattened femoral condyle. Objective To evaluate radiological lucencies of the MFC and their progression in size, in a cohort of Thoroughbred horses, ranging from 5 to 18 months of age and to report on the racing careers of these horses compared to their maternal siblings free of stifle pathology. Study design Retrospective cohort. Methods Radiographic reports were reviewed to identify cases with MFC lucency. Medical data including age at the time of radiographic sale set, sex, and MFC lucency radiographic measurements were recorded. The data were analysed for changes in lucency morphology. Racing data were collected and analysed for the following 5 years. Results From 12 938 sales reports reviewed, 3874 horses were found to have radiographic sets available at both weanling and yearling sales. A MFC lucency ≤3 mm in depth was diagnosed in at least one radiographic sales set in 248 horses (6.4%). The right femur was more commonly affected (73.9%) than the left. Radiographic lucencies in the left femur were significantly smaller (P = .02) than lucencies in the right femur. Radiographic lucencies resolved in 6.1% of cases, 3.6% of cases developed into a cyst, 40.7% of cases were unchanged in size, 23.6% of lucencies decreased in size and 8.2% increased in size. Cysts >3 mm deep regressed into smaller lesions accounting for 4.9% of the lucencies, and 12.9% of lucencies developed from a normal or flat medial femoral condyle contour. Horses with a medial femoral condyle lucency had significantly less starts as a 2‐year‐old vs. their maternal siblings (P < .01). Main limitations Data were collected retrospectively. Measurement errors may have occurred due to the measuring tool scale, small size of the defects and/or radiographic position. Treatments between radiographic studies were unknown and could not be accounted for. Observers were not blinded to radiographic sales reports. Conclusions Radiographically diagnosed lucencies in the MFC of immature Thoroughbreds have the potential to fully resolve or develop into a cyst. However, most radiographic lucencies do not change in size. Thoroughbreds with MFC lucencies had less starts as 2‐year‐olds when compared to their maternal siblings free of stifle pathology.

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