Estrogen receptor (ER) status can predict the efficacy of endocrine therapy. However, the predictive significance of the progesterone receptor (PgR) is controversial in an adjuvant setting. Records of 758 ER+ breast cancer patients who received adjuvant tamoxifen (TAM) for 3-5 years were reviewed to evaluate the predictive value of PgR for TAM treatment in ER+/PgR+ and ER+/PgR- groups. By a median of 40 months' follow-up, there was no significant difference between the two groups with regard to disease-free-survival (DFS). On the basis of STEPP analysis showing the tendency of age effect on DFS in both the ER+/PgR- and ER+/PgR+ groups, we classified the ER+ patients into three strata by age (<45, 45-60, and >or=60 years). There was no significant difference in DFS and overall survival (OS) between the two groups in the <45 stratum and the 45-60 stratum. In contrast, the ER+/PgR- group had a worse prognosis in the >or=60 stratum with regard to both DFS (P=0.0484) and OS (P=0.0009). The results suggest that PgR status might be a predictive factor of benefit to be gained from adjuvant TAM for older ER+ patients with regard to DFS and OS. This should take into account older ER+/PgR- patients who tend to be resistant to TAM.