The study describes two cases of polyostotic fibrous dysplasia of the pelvis tending to progress along the lower extremities. The disease is of unknown origin and develops in the period of puberty. Has chronic qualities with remissions which could last for several years. The disease leads to deformities but spontaneous fractures are also possible. It is diagnosed by means of thorough radiologic examinations and histologically. Therapeutical treatment consists of symptomatic therapy and physiatric procedures, while cases of marked deformities and spontaneous fractures require surgical treatment.