Correlation of elevated levels of the lipogenic enzyme, fatty acid synthase (FASE), with advanced stages of some cancers has drawn attention to this enzyme as a possible marker of poor prognosis. Because recent studies have shown that cancer cells are dependent on fatty acid synthetic activity and pharmacologic inhibitors of this enzyme are selectively cytotoxic to cancer cells, expression of FASE also may provide a potential target for intervention in the neoplastic process. To determine the potential usefulness of expression of FASE in the neoplastic process of the lung, we evaluated its pattern of expression immunohistochemically in archival specimens from 60 human lung specimens with squamous cell cancer (SCC) and associated 'preneoplastic' lesions compared with its expression in the normal bronchial epithelium of 60 noncancer specimens. The expression of FASE was significantly higher in SCC associated uninvolved bronchial epithelium (mean = 0.40 ± 0.03, median = 0.38) compared with its expression in the bronchial epithelium of noncancer specimens (mean = 0.18 ± 0.02, median = 0.16) indicating its early expression. We also observed a statistically significant step-wise increase in FASE expression from SCC associated uninvolved bronchial epithelium (mean = 0.40 ± 0.03, median = 0.38) to epithelial hyperplasia (0.58 ± 0.04, median = 0.57) to SCC (1.53 ± 0.06, median = 1.50). The results suggested that expression of FASE is an early event in the development and progression of SCC of the lung. The inhibition of fatty acid synthesis by inhibiting enzymatic function with metabolic analogues may be a useful strategy in the treatment of SCCs. The expression of FASE in early lesions such as SCC associated uninvolved bronchial epithelium and epithelial hyperplasia might also provide a potential means for intervention early in the neoplastic process in the lung or even preventing their malignant transformation to invasive carcinomas. (C) 2000 by W.B. Saunders Company.