Apoptosis and the expression of Fas receptor (Fas) and Fas ligand (FasL) were studied in 8 samples of normal oral mucosa (OM) and in 19 oral squamous cell carcinomas (OSCC) by immunohistochemistry and the TUNEL method. Fas was detected in less than 2% of cells in OSCC compared with 84.3+/-9.0% of cells in the basal layer in OM. FasL was found to be highly expressed in poorly differentiated lesions (90.9+/-3.6%), and in cells of both the basal (88.3+/-4.3%) and central (85.3+/-5.7%) parts of moderately differentiated lesions, whereas in well-differentiated (WD) lesions expression was considerably lower in both basal (42.7+/-4.1%) and central (11.5+/-2.4%) parts. In normal OM FasL was primarily detected in cells of the basal layer, but in a high proportion of cells (84.9+/-4.3%). Apoptotic cell death was greater in OSCC (1.6+/-0.6%) than in OM (0.6+/-0.2%, P<0.05) and was most pronounced in the central part of WD OSCC (2.3+/-0.5%). Our results show that Fas is expressed in low quantities in OSCC and that FasL expression correlates negatively with degree of differentiation and apoptosis in OSCC.