Proliferative lesions of the mammary gland are risk markers and potential precursors for the development of breast cancer in postmenopausal women. In this study we evaluated mammary epithelial proliferation and proliferative lesions in a group of 63 aged postmenopausal macaques randomized by social group to receive one of three experimental diets for 8 months: (1) control; (2) control with 17beta-estradiol (E2) at the human equivalent dose of 1.0 mg per day; and (3) control with the soy phytoestrogen equol (EQ) at the human equivalent dose of 105 mg per day. In normal mammary epithelium, treatment with E2 but not EQ resulted in greater proliferation, epithelial area, and progesterone receptor expression (P<0.05 for all). Mammary lesions included columnar cell change (26/63), columnar cell hyperplasia with and without atypia (13/63), atypical ductal hyperplasia (6/63), and atypical lobular hyperplasia (3/63). Lesions were most common within terminal ductal lobular units. The prevalence of columnar cell hyperplasia (total and atypical cases) was higher in animals treated with E2 compared to control (P<0.05 for both). Compared to normal mammary epithelium, columnar cell lesions (CCLs) showed greater constitutive expression of estrogen receptor-alpha across all groups (P<0.001) and greater expression of progesterone receptor in response to E2 (P<0.01). Independent of treatment, animals with CCLs on histology had greater gene expression of estrogen receptor-alpha and markers of estrogen receptor activity (trefoil factor 1) and proliferation (gene for Ki67 antigen) at a site contralateral to the CCL (P<0.05 for all). These findings demonstrate that the terminal ductal lobular units of the postmenopausal mammary gland contain morphologically distinct cell populations that may hyperrespond to E2 exposure, resulting in specific types of hyperplastic lesions.