Dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulfate (DHEAS) are metabolic intermediates in the production of potent androgens, estrogens and other less well-characterized steroids. DHEA(S)1 and closely related steroid hormones have a variety of immunological effects both in vitro and in vivo in experimental animals and humans. Many of these effects have been demonstrated in animal models where there is little circulating DHEA(S), and the demonstrated effects are generally seen at concentrations of DHEA(S) which are supraphysiological in man. The physiological role of DHEA(S) in the immunological system is unknown. Furthermore, the molecular mechanism of action of DHEA(S) is unclear. In this review, I focus on studies of the immunological effects of DHEA(S) and closely related steroid metabolites and analogs, mainly derived from literature published in the last five years. My purpose is to describe the demonstrated effects and to highlight some of the remaining major research issues in this field. These issues include defining the molecular mechanism of DHEA(S) action; determining whether the effect of DHEA(S) is related to the steroid itself or to a metabolic product of DHEA; determining the relationship of physiological function to the pharmacological effects; and determining the molecular basis for species-specific differences in effects.