The mare had lameness and asymmetry edematous on its tarsal and metatarsal joints at the initial physical examination. The pain was elicited with a palpation along the metatarsal articulations. No significant abnormalities were detected in the screening test. However, thermographic images revealed a significant increase in the surface temperature at the joint of the hindlimb when compared to the reference range. At necropsy, an irregularity of the surface and excessive synovial fluid were observed on the right tarsal joint. No bacterial growth was shown in the cultures of synovial fluid. Staphylococcus aureus was detected in the subcutaneous discharge. Taken together, the thermography images were very useful in localizing the area of injury and were an effective diagnostic methodology for assessing lameness.