Background: Although young age breast cancer represents poor prognosis, no definitive explanation could have been made for the phenomenon. A tumor proliferation marker Ki67 is known to be a marker for both prognosis and prediction for chemotherapy responsiveness, and its level varies widely depending on the breast cancer subtype. This study was aimed to analyze Ki67 in relationship with age in hormone receptor positive breast cancer patients. Methods: We retrospectively reviewed 9061 consecutive cases of hormone receptor positive invasive breast cancer from data base at Seoul National University Hospital (SNUH) (between 2000 and 2012), Samsung Medical Center (SMC) (between 2004 and 2010), and National Cancer Center (NCC) (between 2001 and 2010) in Korea. Patients with estrogen receptor (ER) or progesterone receptor (PR) positive tumors were included irrespective of HER2 amplification. A multicenter data of Ki67 level identified by immunohistochemistry (IHC) and age at diagnosis were analyzed. Patients who underwent neoadjuvant systemic therapy were excluded. Results: Total 6222 cases from SNUH, 976 from SMC and 1863 from NCC were included. The three datasets were analyzed separately due to variable IHC methods in each institute. Mean ages were 49.30 years (range 20-86), 47.75 years (range 22-81) and 45.31 years (range 25-59), and mean Ki67 levels were 4.66% (range 1-100), 22.98% (range 1-97) and 14.58% (range 1-90), at SNUH, SMC, NCC respectively. Ki67 level was inversely proportional with age at diagnosis in all three datasets, and the level was significantly higher for patients <40 years compared to ≥40 years (mean Ki67: 5.97 vs 4.41, p<0.001; 28.60 vs 21.88, p<0.001; 17.01 vs 14.03, p<0.001, respectively). There was an inverse relation with age as well when Ki67 level was categorized into ‘<10% vs ≥10% (p<0.001)’, ‘<20% vs ≥20% (p=0.03)’ and ‘<14% vs ≥14% (p<0.001)’ respectively. Conclusions: Despite the variability of assessing Ki67 expression, Ki67 level was significantly higher in young age hormone receptor positive breast cancer from all three analyses. This could partly explain the poor prognosis and substantial responsiveness to chemotherapy in this age group of patients.