Fracture of the patella associated with bilateral osteochondrosis of the superior pole of the patella in a 14-week-old cat is reported with histological findings. Patellar osteochondrosis has been described in humans, horses, pigs, and dogs and is characterised by incomplete union of the ossification centres related to an abnormal process of endochondral ossification. However this disease has not yet been described in cats. Macroscopically, two main fragments separated by interposed tissue were identified on the left patella. In contrast, no fracture but only a fissuration of the articular cartilage was observed on the right patella. Bilateral partial patellectomy was performed. Histological examination of the excised fragments from the left patella revealed two main areas of trabecular bone separated by a wide irregular band of hyaline cartilage. The microscopic aspect of the right patella was similar to that of the left. Serial sections showed the initial appearance of an area of necrosis in the central band of hyaline cartilage, and that this hyaline cartilage was subsequently replaced by fibrovascular connective tissue. These findings indicate that some patellar fractures may be due to patellar osteochondrosis.