This paper describes 17 cats that developed an idiopathic necrosis of the femoral neck. In four cats the lesions were bilateral when they were first examined and five cats developed lesions in the other limb within five months. They were all male cats, two years old or younger, and 15 had been neutered. The initial sign was a vague lameness which typically progressed, often acutely, to a more severe lameness. Radiography demonstrated radiolucency and loss of definition within the proximal femoral metaphysis, the femoral neck. In 12 cases there was a complete radiolucent line across the femoral neck. An excision arthroplasty was carried out on all the affected hips and the lameness resolved in all cases. The clinical and radiological signs suggest a primary bone resorption with secondary fracture of the femoral neck. The lesions have some similarities with Legg-Calve-Perthes' disease, traumatic fracture of the femoral neck, canine metaphyseal osteopathy, bacterial osteomyelitis and experimental feline herpes virus osteomyelitis.