Interest in the health-related effects of soy isoflavones has surged in recent years (Setchell 1998, Setchell and Cassidy 1999). Numerous dietary intervention studies have been performed in areas related to cholesterol lowering and cardiovascular effects, effects on bone, and use as an alternative to conventional hormone replacement for postmenopausal women; these studies have given conflicting results. The targeted intake of isoflavones from soy foods has been derived empirically because there are no guidelines for optimal levels of intake. A daily isoflavone intake ≥50 mg has generally been used in clinical studies, largely on the basis of the early observation that a daily intake of 45 mg causes endocrine modulation of the menstrual cycle of premenopausal women (Cassidy et al. 1994). Few dietary intervention studies have attempted to quantify intake or confirm compliance from blood measurements of isoflavones, and the effect of varying dietary intakes of phytoestrogens is largely unknown. Concerns have been expressed that too much isoflavone in the diet may have negative effects, yet there is a paucity of information on the pharmacokinetics of isoflavones or on how varying dietary intake will influence the circulating levels and bioavailability of these bioactive phytoestrogens.