This thesis is an investigation of the incidence and severity of osteoarthritis (OA) in the metacarpophalangeal joint of Ontario Thoroughbred racehorses. Fifty Thoroughbred racehorses that were euthanized or died within sixty days of racing were collected. Metacarpophalangeal joints were assessed using a macroscopic scoring system incorporating India ink staining, as well as histological scoring, polarized light microscopy and cartilage biochemistry. One quarter of 2- and 3-year old horses had full thickness cartilage lesions and OA severity increased with age. Significant (p<.05) Spearman correlations were found between macroscopic grade and glycosaminoglycan depletion, average polarized light intensity, and India ink uptake. The macroscopic system devised for this study showed good correlations with quantitative methods though the major limitation was insufficient histology sampling. The osteoarthritis identified in young horses would be career limiting, resulting in year to year attrition that would strongly bias any estimate of incidence of OA in older horses.