Imbalance in Th1 and Th2 subsets and their derived cytokines seems to be involved in the immune abnormalities underlying UC and CD. CD30 is a member of the tumour necrosis factor/nerve growth receptor superfamily expressed on T cells producing Th2 cytokines and released as a soluble form. In this study high levels of soluble CD30 were found in sera of UC patients independently of disease activity. Furthermore, increased titres of soluble CD30 molecule were shown, in the same patients, by mitogen-stimulated cultures of peripheral blood mononuclear cells. Our data seem to indicate that an activation of Th2 immune response is involved in the pathogenesis of UC, but not of CD. Furthermore, this finding indicates that serum soluble CD30 measurement may be helpful for differentiating these two forms of inflammatory bowel disease.