There have been no detailed ultrasonographic descriptions of injuries of the accessory ligament of the deep digital flexor tendon (ALDDFT) in the carpal and proximal metacarpal and tarsal and proximal metatarsal regions. The aims of this study were to describe the: 1) normal ultrasonographic appearance of the proximal aspect of the ALDDFT in fore- and hindlimbs and 2) clinical and ultrasonographic features of proximal injuries of the ALDDFT in both fore- and hindlimbs. The proximal aspects of the ALDDFT of both fore- and hindlimbs of 10 horses free from lameness were examined ultrasonographically. Clinical records of 12 horses examined between January 2006 and December 2010 with proximal injuries of the ALDDFT were reviewed. All 4 horses with proximal lesions of the ALDDFT in a forelimb had no localising clinical signs and diagnostic analgesia was required to localise the source of pain. Eight horses with recent onset hindlimb lameness had oedematous swelling in the proximoplantaromedial aspect of the metatarsal region (n = 7) or more generalised thickening in the region of the proximal aspect of the ALDDFT (n = 1). Diagnosis was confirmed ultrasonographically in all horses. One horse was retired for unrelated reasons. One of 3 horses with forelimb lameness returned to full athletic function. Four of 6 horses with hindlimb lameness returned to work; the remaining 2 horses resumed work, but experienced recurrent injury of the ALDDFT. Proximal injuries of the ALDDFT should be considered as a cause of either fore- or hindlimb lameness, even in the absence of gross palpable abnormalities.