CD30 is one of the members of the tumor necrosis factor receptor superfamily, originally described as a marker of Reed-Sternberg and Hodgkin's cells in Hodgkin's lymphoma. CD30 appears to be preferentially expressed on, and its soluble form (sCD30) released by, CD4+ and CD8+ T cell clones capable of producing T helper 2 (Th2)-type cytokines. In noneoplastic conditions, CD30+ T cells are barely detectable in vivo; however, a few allergen-specific CD4+CD30+ T cells inducible to the production of Th2-type cytokines could be sorted out from the circulation of allergic subjects after allergen exposure. Moreover, high numbers of CD30+ T cells were found in the lymph node of a patient suffering from Omenn's syndrome, a rare congenital Th2-mediated immunodeficiency disorder. More importantly, high serum levels of sCD30 were observed in some conditions in which a pathogenetic role for Th2 cells has been suggested, such as Omenn's syndrome, atopy, systemic lupus erythematosus, and after infection with measles virus or human immunodeficiency virus. Thus, detection of CD30+ T cells and/or of increased levels of sCD30 may reflect the presence of immune responses or immune alterations characterized by the prevalent activation of Th2-like cells.