Both subtypes of estrogen receptor (ER), ERα and ERβ, are normally present in the mammary gland. The role of ERα as a prognostic marker in breast cancer is well established due to the beneficial effect of providing tamoxifen as adjuvant therapy. The role of ERβ, however, is less clear. To gain insight into the importance of ERβ in breast cancer, 145 primary breast cancers were examined by immunohistochemistry for ERβ, and the expression level was compared with ERα and progesterone receptor (PR) status. Especially, we wanted to examine the significance of ERβ in the contrasting ERα+/PR+ and ERα−/PR− subgroups. In the ERα+/PR+ subgroup (dual positive), the survival difference between patients with low, medium and high ER β level was statistically significant (p = 0.004), with more than 70% of patients with medium and high ERβ levels surviving 100 months, compared with less than 30% in the group with low ERβ level. Further, for ERα+/PR+ patients there was a reduced risk of fatal outcome by multivariate analysis with increasing ERβ levels (p(trend) < 0.01 [univariate analysis]; p(trend) = 0.05 [multivariate analysis]). The risk was 31% and 27% for medium and high ERβ levels, respectively, compared with low ERβ level, adjusting for standard prognostic factors such as tumor diameter, nuclear tumor grade (quantified by mean nuclear area), lymph node status, and patient age at operation. For patients with ERα−/PR− tumors (dual negative), however, there was no association between ERβ levels and patient outcome. Our findings indicate that ERβ expression provides independent prognostic information for breast cancers with ERα/PR-positive status, a feature typical among screen-detected breast cancers. The role of ERβ needs to be further evaluated especially in this group of breast cancers.