PURPOSE: The Cancer and Leukemia Group B (CALGB) 9343 trial randomized elderly women with estrogen receptor-positive, stage 1 breast cancer to receive either hormone therapy (HT) or HT and radiation therapy (HT-RT) after lumpectomy and showed no difference in survival. After this publication, a practice guideline made HT alone a standard treatment option in British Columbia. The subsequent population-based pattern of practice was studied. METHODS AND MATERIALS: All women aged 70 to 79 years, referred after lumpectomy from 1999 to 2009, with stage 1, estrogen receptor-positive breast cancer were identified. The use of adjuvant HT or HT-RT was evaluated in 2 eras: before (1999-2003) and after (2005-2009) the CALGB publication. The proportion receiving each treatment in these eras was determined. Kaplan-Meier analyses with Cox regression were used for survival endpoints. The Fine and Gray method was used with non-breast cancer death as a competing risk for event-free survival. The Charlson score was used to quantify comorbidity. Adherence was defined as dispensation of 80% of prescribed HT. RESULTS: HT-RT was used in 91% of patients before (n=319) and 89% of patients after (n=403) the CALGB publication (P=.4). In the HT-alone group, the rate of HT adherence was 75% at 1 year and 55% at 4 years. The 10-year locoregional recurrence-free survival rate was 98% with HT-RT and 90% with HT alone (P=.01), whereas the 10-year breast cancer-specific survival rate was 96% with HT-RT and 95% with HT alone (P=.2). Patients with grade 3 histology or lymphovascular invasion were more likely to have low event-free survival. On multivariate analysis, treatment type did not predict overall survival (P=.3). CONCLUSIONS: Our patient outcomes closely matched those of the CALGB trial, suggesting that its results are generalizable to a population of elderly breast cancer patients with typical HT adherence. The CALGB trial results, as well as the resulting practice guideline, did not change the use of adjuvant HT-RT in our population-based cancer program.