Intraepithelial carcinoma contiguous with invasive squamous-cell carcinoma is a conspicuous feature of esophageal cancer. However, whether the mechanism of intraepithelial spreading is due to cell proliferation or field carcinogenesis has yet to be clarified. This study investigated the mechanism of intraepithelial spreading by measuring the cell proliferative activity using argyrophilic nucleolar organizer region (AgNOR) and proliferating cell nuclear antigen (PCNA)-positive cell counting. We examined the AgNOR number and PCNA-positive ratio (PCNA ratio) in the center and outer edge of intraepithelial carcinoma and in the center and deep margin of invasive squamous-cell carcinoma of the esophagus in 50 specimens from 18 cases of esophageal squamous-cell carcinoma concomitant with contiguous intraepithelial carcinoma. The proliferative activity was thus found to differ between the normal epithelium and cancerous lesions (p < 0.001), between intraepithelial carcinoma and invasive cancer (p < 0.001) and between deep margin and center areas of invasive cancer (p < 0.005). On the other hand, such activity was observed to be similar in the center and outer edge of the intraepithelial spread. These findings suggest that cell proliferation is the main mechanism of tumor progression at the invasive site of cancer, whereas in intraepithelial carcinomatous areas, "field carcinogenesis" or a paracrine mechanism, and not cell proliferation, is thought to be the cause of intraepithelial spread of esophageal cancer. These results therefore support the concept of field carcinogenesis.