CD30 is a member of the tumor necrosis factor (TNF)/nerve growth factor (NGF) receptor superfamily, and was originally described as a marker of Hodgkin's and Reed-Sternberg cells in Hodgkin's lymphoma. CD30 is preferentially expressed on CD4+ and CD8+ T-cell clones that produce T helper 2 (Th2)-type cytokines, and is also released in a soluble form by these cells. Elevated serum levels of soluble (s)CD30 have been found in some conditions in which a pathogenic role for Th2 cells has been suggested, such as atopy, Omenn's syndrome, systemic lupus erythematosus, as well as following infection with measles virus or human immuno-deficiency virus (HIV). Here, Gianfranco Del Prete and colleagues suggest a complex and fascinating link between the expression and release of CD30, and the immunopathogenesis of HIV infection.