Breast cancer incidence and mortality rates are markedly lower in the south than in the north of Europe. This has been ascribed to differences in lifestyle and, notably, dietary habits across European countries. However, little information exists on the influence of different dietary regimens on estrogens and, hence, on breast cancer risk. Here we report results of our MeDiet Project, a randomized, dietary intervention study aimed to assess the effect of a Mediterranean diet on the profiles of endogenous estrogens in healthy postmenopausal women. Out of the 230 women who initially volunteered to participate in the study, 115 were found to be eligible and were enrolled. Women were then randomly assigned into an intervention (n = 58) and a control (n = 57) group. Women in the intervention group adhered to a traditional, restricted Mediterranean diet for 6 mo, whereas women in the control group continued to follow their regular diet. Women in the intervention group changed their dietary regimen substantially, and this eventually led to a shift from a prevalent intake of animal fat and proteins to a prevalent intake of vegetable fat and proteins. Regarding urinary estrogens, no significant difference was observed between the intervention and control groups at baseline. After 6 mo, however, control women did not show any major change but women in the intervention group exhibited a significant decrease (over 40%) of total estrogen levels (P < 0.02). The largest part of this modification was based on a marked decrease of specific estrogen metabolites, including hydroxy- and keto-derivatives of estradiol or estrone. To our knowledge, this is the first report to show that a traditional Mediterranean diet significantly reduces endogenous estrogen. This may eventually lead to identify selected dietary components that more effectively decrease estrogens levels and, hence, provide a basis to develop dietary preventive measures for breast cancer.