Aims <p>This study aimed to quantify the prevalence of frailty, and investigate the impact of frailty on adverse outcomes, in older patients with cancer in Vietnam.</p> Methods <p>A prospective, observational study was conducted in adults aged 65 or above with cancer who attended the outpatient clinics of two urban hospitals in Vietnam from September 2023 to May 2024. Frailty was defined by the Carolina Frailty Index (CFI) and participants with a CFI &gt;0.35 were identified as frail. All participants were followed up for 3 months after discharge, recording falls, all-cause hospitalization, and all-cause mortality.</p> Results <p>There were 379 participants (mean age 72.3 years, 48.5% female). The prevalence of frailty was 26.6% (95%CI 22.2% - 31.0%), highest in participants with stomach cancer (35.7%) and lung cancer (33.9%). Participants with advanced stages of cancer had a significantly higher prevalence of frailty: 39.3% in stage 4, 21.7% in stage 3, compared to 18.1% in stage 2 and 13.0% in stage 1. During the follow up, 19.0% of the participants had a fall (44.4% in the frail vs. 9.7% in the non-frail, p&lt;0.001), 33.4% were admitted to hospitals (42.2% in the frail vs. 30.1% in the non-frail, p=0.026). The mortality rate was 1.9% (5.1% in the frail vs. 0.7% in the non-frail, p=0.017). Odds ratios were 7.48 (95%CI 4.24 – 13.40, p&lt;0.001) for falls, 1.71 (95%CI 1.06 – 2.75, p=0.027) for all-cause hospitalization, and 7.10 (95%CI 1.36 – 37.22, p=0.020) for all-cause mortality.</p> Conclusion <p>Frailty was common among the participants, with the highest prevalence among those with stomach and lung cancer. Frailty significantly increased the odds of falls, hospitalization, and mortality in three months post-discharge. Further research is needed to gain a better understanding of the impact of frailty on adverse outcomes, and the quality of life for older adults with cancer in Vietnam.</p>