Abstract: Objective : To investigate the predictive value of progesterone receptor (PR) status for adjuvant endocrine therapy in estrogen receptor-positive (ER+) breast cancer patients, especially the aged. Methods : The records of 768 ER+ patients with breast cancer who received adjuvant tamoxifen for 3 to 5 years were reviewed to evaluate the predictive value of PR status for tamoxifen treatment in ER+/PR+ and ER+/PR- populations with a mean follow-up time of 40 months. Results : The postmenopausal patients accounted for a high proportion of the ER+/PR- population. The proportion of ER+/PR- patients was especially low in those below age 45 and was high in the aged (≥60) compared to ER+/PR+ patients, but there was no statistical significance. With a follow up of a mean of 40 months, there was no difference in disease-free survival (DFS) between the ER+/PR+ group and the ER+/PR- group (P=0.1374). However, the ER+/PR+ group had a better overall survival (OS) (P=0.0102). If the ER+/PR+ group was classified into 3 strata by age (45 years,45-60 years and ≥60 years), both DFS and OS showed no significant difference between the ER+/PR+ group and the ER+/PR-group in those below age 45 or between 45 to 60 years. In contrast, the difference in the group older than 60 was significant for both DFS (P=0.0484) and OS (P=0.0009). Similar Results were found if the groups were classified by menses status. Multivariate analysis supported PR status as an independent prognostic value for overall survival in ER+ patients (all populations: P=0.032; ≥60 years: P=0.006). Conclusions : PR status is a predictive factor for the effect of adjuvant tamoxifen in aged ER+ patients, with a slight predictive value for all ER+ patients. It should be taken into account especially for ER+/PR- elderly patients who tend to be resistant to tamoxifen treatment.