Macroscopic, microscopic and radiological examinations of the navicular bones of 83 horses and ponies showed that the navicular bone in the adult horse has 2 principal routes of blood supply. One supply is present at birth, the other develops within the first 2 to 3 years of life, in response to increasing activity. The size of the nutrient foramina, as seen on radiographs, is related to the type, frequency and regularity of work done by the horse. These foramina are normally conical in shape, alteration to a circular, or mushroom-shape being evidence of occlusive vascular disease in the navicular bone. These changes are visible on a standard antero-posterior radiograph of the navicular bone. A common cause of lameness in the horse is ischaemia of the navicular bone, due to progressive arterial occlusion. Occlusion of at least 2 primary arteries, and some involvement of the compensating collateral blood supply must be present before lameness results.