The disease is most common at age 6 to 9, tends to affect boys, but is more severe in girls. It can be familial. The symptoms include hip and thigh pain, stiff hip, a limp, and diminution in size of the thigh. Over a period of 18 to 24 months the blood supply usually reestablishes itself. During this period, the bone is soft and liable to fracture under pressure, causing collapse of the head of the femur.