PURPOSE: In order to elucidate the tumor angiogenesis and to evaluate the usefulness of some cytokines as parameters of tumor progression and metastasis, we investigated the expression of Factor VIII related antigen (Factor VIII R-Ag) and measured the plasma levels of interleukins-1alpha (IL-1alpha), 1beta (IL-1beta), and 6 (IL-6) during rat liver tumorigenesis. METHODS: Sprague-Dawley rats were given diethylnitrosamine (DEN) by water, and sequentially sacrificed. For evaluation we performed light microscopy, immunohistochemical study using avidin-biotin complex, and ELISA. RESULTS: During the carcinogenesis, the staining of Factor VIII R-Ag was negative in normal livers, focal in hyperplastic nodules, scattered in hepatocellular adenomas and diffusely positive in hepatocellular carcinomas. The serum level of IL-1 GGGalpha peaked at 9 weeks, that of IL-1beta steadily increased to peak at 12 weeks, and that of IL-6 continuously increased after DEN administration. In addition, the serum concentration of IL-6 was markedly elevated in hepatocellular carcinomas metastasized to the lung. CONCLUSION: From the above results it could be suggested that sinusoidal phenotypic acquisition of Factor VIII R-Ag is linked to the development of hepatocellular carcinomas and that serum levels of IL-1beta and IL-6 are related to the multistep development of hepatocellular carcinoma. Furthermore, excess production of IL-6 may be a useful parameter to predict metastatic potentials.