Sixteen cases of osteogenic sarcoma have been found among 1,753 examples of Paget's disease of bone seen at the Mayo Clinic. A clinical and pathological study of these cases has been presented. The English-language literature has been reviewed, and a total of 128 cases have been discussed and evaluated as a group. The incidence of sarcoma among our 1,753 patients with Paget's disease was 0.9 per cent. The average age of patients in whom the two conditions coexisted was fifty-eight years with a range from thirty-two to seventy-two years. The sex ratio of 1,753 patients with Paget's disease was two to one, males to females, and this ratio was the same in the 128 collected cases of sarcoma in Paget's disease. Pain, swelling or both were present in every case of sarcoma in Paget's disease, and these were the most important clinical features in the diagnosis. Many patients were unaware of the presence of Paget's disease until sarcoma developed. Of the sarcomata iii Paget's disease of the long bones, 18 per cent developed at the site of recent fractures, which may or may not have preceded the onset of sarcoma. All ten of the sarcomata examined microscopically in this series were osteogenic sarcomata. Imi five of the seven cases in which Paget's disease was also noted microscopically, very cellular areas were seen, areas which might be termed presarcomatous. The prognosis of sarcoma in Paget's disease is poor, only one five-year survival being found among 128 cases reported in the literature. Treatment has consisted in amputation or roentgen therapy. Delay in diagnosis and treatment is considered to be one of the factors in the poor prognosis, and suggestions for improvement are made.