The etiology of idiopathic portal hypertension (IPH) is unknown, although many studies have suggested that it might be an autoimmune disease. The autologous mixed lymphocyte reaction (AMLR) involves the proliferation of T lymphocytes when co-cultured with autologous non-T cells and may reflect immune control mechanisms in vivo. The AMLRs in the spleen and peripheral blood of three patients with IPH were measured and it was shown that the AMLRs both in the spleen and peripheral blood were significantly suppressed compared to those of normal healthy subjects. By allogeneic MLR, there was a tendency that the disturbance of non-T cells was more intensive than that of T cells. The AMLR of peripheral blood did not improve by splenectomy. Thus, the depressed cause of AMLR in patients with IPH was suggested mainly to disturbance of the antigen-presenting ability of non-T cells, and it was suggested that not only the spleen cells, but systemic immune disturbance caused the impairment of AMLR in IPH.