PURPOSE: Luminal breast cancer has a long natural history, with recurrences continuing beyond 10 years after diagnosis. We analyzed long-term follow-up (LTFU) of efficacy outcomes and adverse events in the Breast International Group (BIG) 1-98 study reported after a median follow-up of 12.6 years. PATIENTS AND METHODS: BIG 1-98 is a four-arm, phase III, double-blind, randomized trial comparing adjuvant letrozole versus tamoxifen (either treatment received for 5 years) and their sequences (2 years of one treatment plus 3 years of the other) for postmenopausal women with endocrine-responsive early breast cancer. When pharmaceutical company sponsorship ended at 8.4 years of median follow-up, academic partners initiated an observational, LTFU extension collecting annual data on survival, disease status, and adverse events. Information from Denmark was from the Danish Breast Cancer Cooperative Group Registry. Intention-to-treat analyses are reported. RESULTS: Of 8,010 enrolled patients, 4,433 were alive and not withdrawn at an LTFU participating center, and 3,833 (86%) had at least one LTFU report. For the monotherapy comparison of letrozole versus tamoxifen, we found a 9% relative reduction in the hazard of a disease-free survival event with letrozole (hazard ratio [HR], 0.91; 95% CI, 0.81 to 1.01). HRs for other efficacy end points were similar to those for disease-free survival. Efficacy of letrozole versus tamoxifen for contralateral breast cancer varied significantly over time (0- to 5-, 5- to 10-, and . 10-year HRs, 0.62, 0.47, and 1.35, respectively; treatment-by-time interaction P = .005), perhaps reflecting a longer carryover effect of tamoxifen. Reporting of specific long-term adverse events seemed more effective with national registry than with case-record reporting of clinical follow-up. CONCLUSION: Efficacy end points continued to show trends favoring letrozole. Letrozole reduced contralateral breast cancer frequency in the first 10 years, but this reversed beyond 10 years. This study illustrates the value of extended follow-up in trials of luminal breast cancer.