REASONS FOR PERFORMING STUDY: Arthrosis of the articular process joints (APJs) in the caudal thoracolumbar region of horses may cause back pain and subsequent reduced performance or lameness. Ultrasound-guided injections of the APJs of the equine back have been described only briefly in the literature. OBJECTIVES: To evaluate factors affecting the accuracy of intra-articular injections of the APJs in the caudal thoracolumbar region. METHODS: One-hundred-and-fifty-four injections with blue dye were performed on APJs including the T14-L6 region in 12 horses subjected to euthanasia for reasons unrelated to back problems. The backs were subsequently dissected to verify the location of the injectate in relation to the APJs. RESULTS: Twenty-seven percent of the injections were found to be intra-articular and a total of 77% found to be within 2 mm of the joint capsule including the intra-articular deposits. Application of a medial approach and 18 gauge needle were significantly associated with an intra-articular injection or deposition close to the joint capsule. Operator, APJ (location) and back number (chronological) did not significantly affect the accuracy of injection. CONCLUSIONS AND POTENTIAL RELEVANCE: Injection of the vertebral APJ in the thoracolumbar region using ultrasound guidance is a reliable method, as most of the injections were either in or within 2 mm of the joint. Based on the findings of this cadaver study, the medial approach is expected to be the most accurate in live horses. Further investigations are required to evaluate the diagnostic and therapeutic potential of this method in clinical practice.