In our previous study, we suggested that long-standing pleural inflammation might be an etiological factor for development of pleural soft-tissue sarcomas, especially malignant fibrous histiocytoma and angiosarcoma (AS). To study the etiological importance of chronic pyothorax for development of pleural AS, a nationwide study of AS in Japan was carried out. Histological and clinical findings in 99 collected cases with AS were reviewed. Six (6%) of the 99 cases were chronic pyothorax-associated AS. Another three cases of pyothorax-associated AS previously reported by us were also included. They were eight males and one female from 45 to 74 yr of age (median 65 yr). All patients had a 15- to 40- (mean 30) yr history of chronic tuberculous pyothorax. Histologically, the tumors contained irregular, occasionally dilated vascular channels, which were invested by tumor cells with an epithelioid appearance. From the statistical data reported previously, annual incidence of AS is probably 0.01 to 0.02 per 100,000 population. Meanwhile, there was an incidence rate of pyothorax-associated AS among chronic pyothorax patients of 0.036 per 100 patients. Thus, the frequency rate of pyothorax-associated AS is supposedly over 3600-fold higher than that in normal population. The present study shows that chronic tuberculous pyothorax is one of the causes of AS development.