Purpose: The management of older adults with breast cancer (BC) remains controversial.The challenging assessment of aging idiosyncrasies and the scarce evidence of therapeutic guidelines can lead to undertreatment. Our goal was to measure undertreatment and assess its impact on survival. Methods: Consecutive patients with BC aged 70 years or older were prospectively enrolled in 2014. Three frailty screening tools (G8, fTRST, and GFI) and two functional status scales (Karnofsky performance score and Eastern Cooperative Oncology Group Performance Status) were applied. Disease characteristics, treatment options, and causes of mortality were recorded during a 5-year follow-up. In addition, we defined undertreatment and correlated its survival impact with frailty. Results: A total of 92 patients were included in the study. The median age was 77 (range 70– 94) years. The prevalence of frailty was discordant (G8, 41.9%; fTRST, 74.2%; GFI, 32.3%).Only 47.8% of the patients had a local disease, probably due to a late diagnosis (73.9% based on self-examination). Thirty-three patients (35.6%) died, of which 15 were from BC.We found a considerably high proportion (53.3%) of undertreatment, which had a frailtyindependent negative impact on the 5-year survival (hazard ratio [HR], 5.1; 95% confidence interval [CI], 2.1–12.5). Additionally, omission of surgery had a frailty-independent negative impact on overall survival (HR, 3.9; 95% CI, 1.9–7.9). Conclusion BC treatment in older adults should be individualized. More importantly, assessing frailty (not to treat) is essential to be aware of the risk-benefit profile and the patient's well-informed willingness to be treated. Undertreatment in daily practice is frequent and might have a negative impact on survival, as we report.