The aim of this study was to analyze the effect of transdermal estradiol replacement therapy (HRT) on immune function in menopausal women.A prospective comparative study was carried out in 30 women, aged 48-55 years, who were divided into two groups; 20 of them received transdermal estradiol 50 microg/day during 3 months and 10 who refused to receive HRT served as controls. Serum interleukins were quantified by specific immunoenzymatic assays; in addition, hormones of somatotropin axis and prolactin (PRL) were quantified by IRMA and RIA.Baseline elevated interleukin (IL)-6 levels decreased significantly (p < 0.001) after transdermal HRT as compared with the nontreated group. Contrarily, IL-2 and IL-10 levels as well as mitogenic induced T-cell proliferation were unchanged under HRT. Insulin-like growth factor-I, growth hormone and PRL levels were unaltered by transdermal HRT.Decrement of IL-6 in parallel with absent effect on some indices of immune activity suggests a beneficial action of transdermal HRT. These findings contrast with those demonstrating an increment of immune response in women taking oral HRT. Thus, the route of administra tion determines the effect of HRT on immune function.Copyright 2002 S. Karger AG, Basel