The authors studied 11 pediatric intrathoracic neoplasms that share clinicopathologic features and constitute a specific tumor in children. These neoplasms were intrapulmonary, mediastinal, or pleural-based masses. A common histologic feature was the presence of small, primitive cells with blastematous qualities separated by an uncommitted stroma. Focal rhabdomyosarcomatous, chondrosarcomatous, and liposarcomatous differentiation was observed. Epithelial components had bland cytologic features and probably represented entrapped benign epithelium and/or mesothelium. The prognosis for these patients was grave; seven patients died of their disease 5 months to 2 years after diagnosis. Two patients have survived disease-free for 10 and 12 years after diagnosis. Two recent cases are alive 14 and 32 months after diagnosis. This neoplasm constitutes a distinct entity which has been reported in the literature as pulmonary blastoma in children. It differs from pulmonary blastoma in adults because of its variable anatomic location, primitive embryonic-like blastema and stroma, absence of a carcinomatous component, and potential for sarcomatous differentiation. The designation of pleuropulmonary blastoma is suggested by the authors for these intrathoracic neoplasms of childhood rather than pulmonary blastoma for histogenetic and anatomic reasons. The clinicopathologic features, immunophenotypic and ultrastructural characteristics, possible histogenesis, and differential diagnosis of these neoplasms from other thoracopulmonary tumors in children serve as the basis for this report.