Differences in breast tissue can be determined using breast-enhanced scintigraphy test (BEST) imaging. Minimal work in vivo has been done previously to determine the effects of soy protein on breast tissue. The authors' earlier work demonstrated reduction in inflammatory changes in breast tissue. This work was conducted to examine the effect of daily soy protein consumption on a larger group of women over the course of 1 year. Sixty-four premenopausal women were studied after initial BEST imaging evaluation revealed fibrocystic changes of the breast. Women were asked to consume a medical-grade soy protein on a daily basis, making no other dietary or lifestyle changes during that time. Each woman underwent BEST imaging 1 year later with the results compared to the initial findings. Women and their physicians reported a subjective reduction in both breast tenderness and fibrocystic disease (FCD). There was a nonstatistical reduction in both the average and maximal count breast activity following 1 year of daily soy consumption. There was a statistically significant reduction ( P < .01) in variability of tissue activity following 1 year of soy protein treatment. This is the first in vivo study looking at the effect of soy protein on breast tissue health. The findings are promising and showed both objective and subjective findings consistent with a reduction in fibrocystic disease of the breast. Further research is needed to confirm these findings in a greater number of women and to determine if soy protein has the same beneficial effect in atypia and breast cancer.