Genistein, like other phytochemicals, has beneficial health effects, but its bioavailability is limited. This research studied the effect of complexation of genistein with starch on genistein bioavailability. Genistein release from these complexes was tested in vitro under simulated intestinal conditions and in vivo in rats fed high-amylose corn starch (HACS)-genistein complexes (experimental group) as compared to those fed a physical mixture of HACS and genistein (controls). In vitro results showed that genistein release is sustained and fits the normal transit time of food in the intestine. The genistein concentration in the plasma was twice as high in the experimental group versus controls; the genistein concentration in the urine was also higher in the experimental group but lower in the feces. These results indicate that starch-genistein complexes increase genistein bioavailability and suggest that starch can affect the bioavailability of additional food components.