Key PointsQuestionWhat is the long-term survival associated with tamoxifen therapy for postmenopausal patients with luminal A or luminal B subtype tumors? FindingsThis secondary analysis of the Stockholm Tamoxifen (STO-3) trial of 462 postmenopausal patients with lymph node-negative breast cancer found that patients with luminal A or luminal B tumor subtypes had a long-term risk of distant metastatic breast cancer and benefited from tamoxifen therapy for 15 years and 5 years after diagnosis, respectively. MeaningPatients with luminal A tumor subtype appeared to have a long-term benefit from tamoxifen therapy, and patients with luminal B subtype appeared to have an early benefit from therapy, when the risk of distant metastatic disease was high. This secondary analysis of the Stockholm Tamoxifen (STO-3) clinical trial, which was conducted from 1976 to 1990, assessed the long-term survival associated with tamoxifen therapy in postmenopausal patients with luminal A or B breast cancer tumor subtypes. ImportancePatients with estrogen receptor (ER)-positive breast cancer have a long-term risk for fatal disease. However, the tumor biological factors that influence the long-term risk and the benefit associated with endocrine therapy are not well understood. ObjectiveTo compare the long-term survival from tamoxifen therapy for patients with luminal A or luminal B tumor subtype. Design, Setting, and ParticipantsSecondary analysis of patients from the Stockholm Tamoxifen (STO-3) trial conducted from 1976 to 1990, which randomized postmenopausal patients with lymph node-negative breast cancer to receive adjuvant tamoxifen or no endocrine therapy. Tumor tissue sections were assessed in 2014 using immunohistochemistry and Agilent microarrays. Only patients with luminal A or B subtype tumors were evaluated. Complete long-term follow-up data up to the end of the STO-3 trial on December 31, 2012, were obtained from the Swedish National registers. Data analysis for the secondary analysis was conducted in 2017 and 2018. InterventionsPatients were randomized to receive at least 2 years of tamoxifen therapy or no endocrine therapy; patients without recurrence who reconsented were further randomized to 3 additional years of tamoxifen therapy or no endocrine therapy. Main Outcomes and MeasuresDistant recurrence-free interval (DRFI) by luminal A and luminal B subtype and trial arm was assessed by Kaplan-Meier analyses and time-dependent flexible parametric models to estimate time-varying hazard ratios (HRs) that were adjusted for patient and tumor characteristics. ResultsIn the STO-3 treated trial arm, 183 patients had luminal A tumors and 64 patients had luminal B tumors. In the untreated arm, 153 patients had luminal A tumors and 62 had luminal B tumors. Age at diagnosis ranged from 45 to 73 years. A statistically significant difference in DRFI by trial arm was observed (log rank, Pamp;lt;.001 [luminal A subtype, n=336], P=.04 [luminal B subtype, n=126]): the 25-year DRFI for luminal A vs luminal B subtypes was 87% (95% CI, 82%-93%) vs 67% (95% CI, 56%-82%) for treated patients, and 70% (95% CI, 62%-79%) vs 54% (95% CI, 42%-70%) for untreated patients, respectively. Patients with luminal A tumors significantly benefited from tamoxifen therapy for 15 years after diagnosis (HR, 0.57; 95% CI, 0.35-0.94), and those with luminal B tumors benefited from tamoxifen therapy for 5 years (HR, 0.38; 95% CI, 0.24-0.59). Conclusions and RelevancePatients with luminal A subtype tumors had a long-term risk of distant metastatic disease, which was reduced by tamoxifen treatment, whereas patients with luminal B tumors had an early risk of distant metastatic disease, and tamoxifen benefit attenuated over time.