Background: Multiple markers are used to assess frailty and vulnerability, conditions associated with the development of chemotherapy toxicity (CTT). However, there is still no consensus on which condition has stronger association with this adverse effect of chemotherapy in the elderly. Objectives: To evaluate the association between frailty and vulnerability with the development of CTT in oncogeriatric patients. Design, Setting, and Participants: Through a retrospective cohort, a secondary database of 496 male oncogeriatric military patients treated at the Geriatrics Service of the Naval Medical Centre of Peru during 2013–2015 was analyzed. Measurements: With prior informed consent, the presence of frailty, assessed by Fried Phenotype; and vulnerability, assessed by the Vulnerable Elders Survey-13 (VES-13) and G-8, was determined. The follow-up of patients in chemotherapy was performed every 8 weeks, to determine the development of CTT (according to the Common Terminology Criteria for Adverse Events v4.0). In addition, we included sociodemographic characteristics, medical background information and functional assessment variables. The data collected was encoded and imported into STATA v14.0 statistical package for analysis. Multivariate analysis was performed using crude and adjusted Cox regression models. The reported measure was the hazard ratio (HR) with their respective 95% confidence intervals (95%CI). Results: The average age was 79.2 ± 4.3 years. 129 (26.01%) developed CTT during follow-up. Similarly, 129 older adults (26.01%) were positive for frailty according to Fried phenotype; 101 (20.36%) were positive for vulnerability with VES-13, and 112 (22.58%) with G-8. In the adjusted Cox model, by type of cancer and adverse effects, a statistically significant association was found between the 3 scales evaluated and the development of CTT, with the Fried Phenotype as the scale with the strongest association (HR=2.01; 95%CI: 1.04–4.90). Conclusions: The frailty and vulnerability in the elderly are conditions associated with the development of CTT. The Fried phenotype was the scale with the most significant association with the outcome studied.