Blood eosinophilia is invariably associated with acute cellular episodes of rejection, characterized by blastogenic inflammation in fine-needle aspiration cytology of the renal transplant. The eosinophilic episode usually precedes the onset of inflammation by 1-4 days and carries no correlation with the intensity of the inflammation. No eosinophilia is present in the blood immediately (within 3 days) after transplantation or in patients displaying neither clinical nor cytological evidence of rejection. This suggests that the up-regulation of eosinophilia is not related to the transplantation procedure itself or to inflammation, but rather to very early steps in the rejection cascade. A likely possibility is a direct signal from alloactivated T cells. Treatment of rejection with steroids, but not with cyclosporine or, when not treating rejection, rapidly down-regulates the episode. At present we do not know whether episodes of eosinophilia are also associated with other occasions of T cell activation, like viral infection-therefore the value of eosinophil differential as a diagnostic test for rejection cannot be determined.