From April 1991 to May 1993, 23 patients entered a phase II clinical study of surgical resection and adjuvant intracavitary photodynamic therapy for malignant pleural mesothelioma. Two days preoperatively, patients received an intravenous injection of 2 mg/kg of the photosensitizer Photofrin. Six patients underwent a pleuro-pneumonectomy, and 15 patients a pleurectomy, after which intracavitary photodynamic therapy was administered. A total light energy dose of 20 to 25 J/cm2 was given. In 2 patients the tumor was unresectable due to intrapericardial invasion. Postoperative complications were noted in more than 50 percent of patients; 2 patients died of postoperative complications. Postoperative survival was analyzed according to intraoperative staging proposed by the American Joint Committee for Cancer Staging, published in 1992. The overall estimated median survival is 12 months; that of stage III and IV patients is 7 months. Five patients with stage I and II diseases (who had grossly complete resection by pleurectomy) are alive, disease-free, for 11, 17, 18, 21, and 33 postoperative months. Intraoperative staging is important in carrying out further clinical studies of malignant pleural mesothelioma.