Idiopathic osteolysis is a very rare bone condition of unknown origin. The length of the illness usually lasts for several years. The purpose of this paper is to point out the possibility of severely rapid progression of idiopathic osteolysis and to contribute to a better understanding of the natural history of Gorham-Stout disease. We report the case of a 58-year old man whose right humeral head and lateral clavicle had disappeared as a result of massive osteolysis observed only six months after onset of the symptoms. Such rapid progression has not been reported previously. His physical examination was otherwise normal. Serum interleukin-6 (IL-6) was elevated and the other laboratory tests were normal. Radiography, computed tomography scan, bone scan, and magnetic resonance imaging (MRI) evaluation revealed extensive destruction of the right humeral head and lateral clavicle involving the acromioclavicular joint. Although the histologic features could not be confirmed because the patient refused a biopsy, it was felt that the patient satisfies the other diagnostic features of Gorham-Stout disease. Follow-up examinations revealed no further bone or soft-tissue involvement. Gorham-Stout disease should be considered in the differential diagnosis of such severely rapid progression of lytic bony lesions.