Idiopathic portal hypertension (IPH), a disorder of unknown etiology, is characterized by a noncirrhotic portal hypertension associated with splenomegaly, hypersplenism, and anemia. We examined the surface phenotypes of T cells and the T cell receptor V beta repertoire in patients with IPH. The T cells in peripheral blood samples and from spleens showed a marked increase in frequencies of HLA-DP(+)- and HLA-DR(+)-activated T cells and the observed high frequencies in the blood were to a considerable extent reduced after splenectomy. Thus, the continuous activation of T cells may occur initially in the spleen. Investigation of T cell receptor V beta repertoire revealed a significant skewing of V beta 9 and V beta 11 in both peripheral blood and splenic T cells and V beta 12 in splenic T cells. The IPH may be a disease mediated by a continuous stimulation with either a certain antigen or more likely a superantigen.