Summary: Different types and locations of fractures of the third metacarpal bone have been discussed for decades, especially in regard to treatment and prognosis. There is lack of documentation of frontal plane third metacarpal fractures diagnosed by MRI within the extensive previous literature. The present case report describes an incomplete frontal plane third metacarpal fracture in a dressage horse. The A 6 year-old Hanoverian mare was suddenly lame during side passes within a dressage contest. Physical examination revealed a grade 3/5 lameness of the left forelimb. When standing the horse showed a tendency to leg rest. A discrete oedematous swelling from the medioproximal metacarpus to the middle third of the flexor tendons was palpable. Rotation of the phalanx resulted in a mild pain response. Initially a basic set of x-ray images was carried out, which did not show an evident fractureline. To further evaluate the cause of local swelling and lameness a standing MRI examination was carried out with a 0.27 Tesla Hallmarq EQ2 dedicated equine MRI system. In consequence, an incomplete frontal plane fracture of the third metacarpal bone was confirmed. The images depict typical features of acute fractures in MR studies. Angle adapted x-ray images were taken and the fracture line initially obscured by the splint bones became visible. No diagnostic block was performed due to the risk to worsen the incomplete fracture. Conservative management was chosen. During the first four weeks immobilisation was applied and exercise was restricted to box rest for another four weeks. During that time the mare was re-evaluated regarding to lameness in the walk, by MRI and by x-ray images. In the present case conservative management led to complete recovery. The last recheck in the stable took place after 46 weeks by the referring veterinarian. Until that time no signs of secondary osteoarthritis had occurred in follow-up radiographs taken by the referring veterinarian.