Purpose Limited data exist on the molecular biology, treatment, and outcomes of breast cancer in men, and much of our understanding in this area remains largely an extrapolation from data in women with breast cancer. Materials and Methods We studied men and women with hormone receptor-positive breast cancer and the 21-gene Breast Recurrence Score (RS) results. Differences in clinical characteristics and gene expression were determined, and distribution of RS results was correlated with 5-year breast cancer-specific survival (BCSS) and overall survival. Results There were 3,806 men and 571,115 women. Men were older than women (mean age, 64.2 v 59.1 years; P < .001). RS < 18 predominated in both genders, but RS ≥ 31 was more frequent in men (12.4% v 7.4%; P < .001), as were very low scores (RS < 11; 33.8% v 22.1%; P < .001). Mean gene expression was higher in men for the estrogen receptor (ER), proliferation, and invasion groups. ER was lowest and progesterone receptor was highest in women younger than 50 years of age, with a progressive increase in ER with age. Men younger than 50 years of age had slightly lower ER and progesterone receptor compared with older men. Survival data were available from SEER for 322 men and 55,842 women. Five-year BCSS was 99.0% (95% CI, 99.3% to 99.9%) and 95.9% (95% CI, 87.6% to 98.7%) for men with RS < 18 and RS 18-30, respectively, and for women, it was 99.5% (95% CI, 99.4% to 99.6%) and 98.6% (95% CI, 98.4% to 98.8%), respectively. RS ≥ 31 was associated with an 81.0% 5-year BCSS in men (95% CI, 53.3% to 93.2%) and 94.9% 5-year BCSS (95% CI, 93.9% to 95.7%) in women. Five-year BCSS and overall survival were lower in men than in women. Conclusion This study reveals some distinctive biologic features of breast cancer in men and an important prognostic role for RS testing in both men and women.