Intrathoracic spread in patients with pseudomyxoma peritonei (PP) is rare. We reviewed 101 patients uniformly treated at our institution for PP of appendiceal origin. In that study, we suggested mucinous carcinoma peritonei (MCP) as the pathologic terminology for all cases of PP. Four patients had pathologically documented pleuropulmonary involvement. We subsequently examined another patient with pleural invasion. Of 5 patients, 3 had low-grade histologic features in the peritoneum; these showed variably proliferative, bland-appearing neoplastic cells arising from low-grade appendiceal mucinous neoplasms. In 2 cases, 1 or more pulmonary parenchymal metastases of low histologic grade developed. The lack of pleural involvement argued against transdiaphragmatic tumor extension. The third patient with low-grade MCP had direct extension through the left diaphragm involving the left pleural and pericardial spaces without pulmonary parenchymal involvement. In the 2 patients with high-grade MCP, right-sided pleural effusions developed. Neither patient had documented injury to the diaphragm. Pleural cytologic examination revealed high-grade adenocarcinoma cells singly, in small clusters, and in large spheres. The smear backgrounds contained wispy mucin. None of the 5 patients developed thoracic lymph nodal metastases. Although rare, mucinous neoplasms from PP may involve the thorax.