The histogenesis of localized fibrous tumor of the pleura (LFTP) is controversial. We studied 12 LFTP's by light microscopy; by immunohistochemical staining for cytokeratin (CK), vimentin, muscle-specific actin, desmin, S-100 protein, epithelial membrane antigen (EMA) and factor VIII; by electron microscopy in 6 tumors; and by lung digestion for asbestos bodies in 4 cases. Three histologic patterns occurred in combination: 1) collagenous, 2) cellular and 3) hypocellular/myxoid. Hemangiopericytoma-like foci were prominent in the cellular areas of 9 tumors. Unusual features included diffuse small cells in 3 tumors, microcystic foci in 2, macrocystic areas in 5 and tumor giant cells in 4 tumors. Neoplastic cells in all patterns stained positively for vimentin and actin in 9 and 4 tumors, respectively, and were negative for all other markers. CK and EMA were identified in mesothelial and epithelial invaginations only. Ultrastructurally, neoplastic cells demonstrated intercellular junctions, intermediate or thin filaments, dense bodies and rough endoplasmic reticulum. Basal lamina was focally present in 5 tumors, while tonofilaments, desmosomes and short microvilli were observed in one case. Our results support the conclusion that LFTP is a neoplasm of the multipotential subserosal cell, and usually expresses mesenchymal (fibroblastic/myofibroblastic) differentiation. Coexpression of mesothelial features is rare. Lung asbestos body quantitation in 4 patients suggests that there is no association between LFTP and asbestos exposure.