Ki-67 proliferation status measured by immunohistochemistry has long been discussed as both a prognostic and predictive indicator in breast carcinoma, yet its clinical robustness remains uncertain. In this analysis, Ki-67 was reassessed retrospectively by three independent pathologists using two scoring strategies—a whole-slide visual estimate and a focused quantitative count at the tumour edge—in a cohort of 411 early breast cancer patients followed for a median of 26.8 years. Agreement among observers was excellent for both scoring approaches. Recurrence risk linked to Ki-67 varied over time: individuals with high proliferation (Ki-67 ≥ 30%) initially showed greater recurrence rates, but after roughly 4.5 years, the lower proliferation category exhibited increased late risk. Among estrogen receptor (ER)-positive cases, the intermediate Ki-67 group behaved similarly to the high-proliferation category early on, then diverged to resemble the low-proliferation outcomes later. ER-positive pN0-1 patients with mid-range Ki-67 who received endocrine therapy alone achieved outcomes comparable to those treated with chemotherapy. A threshold near 20% appeared optimal for separating low- and high-proliferation groups. Overall, straightforward visual scoring of Ki-67 on full slides proved reliable for clinical use, and Ki-67 was reaffirmed as a key prognostic and predictive biomarker.
