Preclinical and clinical data suggest a protective role for estrogens on colon cancer (CRC) risk. estrogen receptor (ER) β is the prevalent ER in normal colonic mucosa, whereas its expression is significantly reduced in CRC. An increased ERα/β ratio has been documented in colon carcinomas and is associated with increased proliferation and decreased apoptosis. The aim of our study was to evaluate the expression of activated ERα and its prognostic implications in patients with stage II-III CRC. Phospho-ERα(Ser167) (pERα(Ser167)) expression was assessed by immunohistochemistry in 218 CRC paraffin-embedded tumor samples. A high pERα(Ser167) expression was more commonly observed in women, older patients, and patients with high baseline glucose levels. This higher pERα(Ser167) expression was associated with decreased 5-year disease-free interval (DFI; 66% versus 78%, P = .07) and overall survival (65% versus 73%, P = .46). The negative impact of high pERα(Ser167) expression on DFI was particularly significant (P < .05) in women (85% versus 60%), young (82% versus 61%), nondiabetic (85% versus 66%), and stage II patients (86% versus 72% and low versus high pERα(Ser167), respectively). Multivariate analysis confirmed that pERα(Ser167) score was a significant prognostic factor for both DFI and overall survival, independent of sex, age, glucose levels, tumor stage, bowel obstruction/perforation, or adjuvant chemotherapy. These findings illustrate the relevance of estrogen pathways in colon cancer biology and may provide novel therapeutic avenues to be explored in this context.