In children, primary tumors of the lung constitute a unique subset of quasineoplastic and unequivocally neoplastic lesions whose benign or malignant potential is not always predictable on the basis of morphologic findings. One such neoplasm in the latter category is the primary bronchopulmonary fibrosarcoma (PBPF). This clinicopathologic, ultrastructural, and immunohistochemical study documents our experience with five PBPFs in newborns and children up to 11 years of age at diagnosis. The tumors were either endobronchial or intraparenchymal in location. A uniform population of interlacing bundles and sheets of densely arranged spindle cells with variable mitotic activity was observed microscopically in each case. Ultrastructurally, the cells had the features of fibroblasts and vimentin was the only immunohistochemical marker identified. Despite the disturbing pathologic findings, the four children with more than 1 year of follow-up have survived well 4 to 9 years after surgical resection. Our results are compared with the 21 cases of PBPF reported in the literature and the differential diagnosis is discussed.