Different isoforms of fibronectin are derived from a single gene by alternative processing of the primary RNA transcript or by posttranslational modifications. We have previously demonstrated that an oncofetal fibronectin (FN) isoform derived by O-glycosylation is highly associated with malignancy in breast and oral tumors. Another oncofetal FN isoform containing the ED-B sequence is derived by alternative splicing, and FN containing ED-B has been found to be a stromal marker of malignancies in various tissues. Here we report a comparative study by immunohistology of the distribution of the ED-B-containing isoform and the oncofetal FN isoform derived by O-glycosylation, in oral squamous cell carcinomas, premalignant lesions, and normal oral mucosa. A selective expression of the ED-B-containing isoform was demonstrated in close relation to the invading carcinoma (38/38), whereas there was virtually no staining in submucosa underlying premalignant lesions (1/11) and normal epithelium (0/5). The ED-B-containing FN showed close co-distribution and staining pattern with the oncofetal isoform derived by O-glycosylation. These results demonstrate that accumulation of FN adjacent to oral carcinomas includes both the ED-B-containing isoform and the isoform derived by O-glycosylation. Although both the change in primary structure and glycosylation of FN create conformational and immunologically detectable changes, the functional consequences in association with invasive carcinoma are poorly understood at present. Diagnostic implications especially of borderline lesions as well as evaluation of tumor aggressiveness may, however, be important.