The transmembrane tyrosine kinase receptor c-met with its ligand, hepatocyte growth factor/scatter factor (HGF/SF), acts as a mitogen, motogen, and morphogen in many normal epithelia. HGF/SF-met signaling has also been implicated in neoplastic progression and metastasis. In this study, immunofluorescence staining and quantitative laser scanning confocal microscopy were used to measure c-met expression in ovarian surface epithelial tumors from 17 oophorectomy specimens. These specimens were from patients aged 25 to 81 (mean age, 52) and included 10 malignant tumors, 4 borderline tumors, and five benign tumors including a Brenner tumor. For comparison, c-met expression was measured in normal tissues from the same patients, including 4 ovarian surface epithelia, 4 fallopian tube epithelia, 2 endometria, and 3 endocervical epithelia, as well as 3 cases of endometriosis. Relative pixel intensity values of c-met expression ranged from 0.4 in a normal ovarian surface epithelium to 22.3 in a borderline serous tumor. Malignant tumors (mean, 9.6) and borderline tumors (mean, 12.9) had higher average c-met expression levels than normal tissues (mean, 3.6) and endometriosis (mean, 1.8). The expression levels of benign tumors were intermediate (mean, 7.9). Among the normal tissues, c-met expression in fallopian tubes (mean, 8.2; range, 3.4-12.9) was higher than that of the other normal epithelia (mean, 1.6; range, 0.4-4.3). In eight cases where both normal and malignant tissues were sampled, c-met expression was significantly greater in malignant than in normal epithelia (P = 0.01). These findings indicate that c-met plays a role in the biology of the normal tissues examined. They confirm that its expression increases in the malignant progression of ovarian surface epithelial tumors, and suggest that increases comparable to those in frankly malignant carcinomas have already been reached in borderline lesions, ie, early in the neoplastic process.