This study was conducted to evaluate the influence of diets containing genistein and soy extract on the growth of MDA-MB-231 cells implanted into female Balb/c mice. Four-week-old female athymic nude mice (Balb/c) were acclimated to an AIN-93G control diet for 1 week and then injected MDA-MB-231 cells ($1{ imes}10^6$/site) and were continued on the on AIN-93G control diet. Five weeks after injecting the MDA-MB-231 cells ($1{ imes}10^6$/site), two experimental groups were assigned to diets containing genistein (750 ${mu}g/g$ AIN-93G diet) or 0.6% soy extract (containing genistein at 750 ${mu}g/g$ AIN-93G diet) until they were sacrificed. Tumor growth was significantly reduced in the groups treated with genistein and soy extract compared to the control group. The results of the proliferating cell nuclear antigen (PCNA) assay also revealed that genistein and soy extract treatment reduced the proliferation of MDA-MB-231 cells in vivo. In the present study, dietary isoflavone was provided just before solid tumor formation, and thus the timing of dietary isoflavone administration may be critical to the suppression of tumor growth.