Abstract Background Standard oncology tools are inadequate to distinguish which older patients are at higher risk of developing chemotherapy-related complications. Materials and Methods Patients over 70 years of age starting new chemotherapy regimens were prospectively included in a multicenter study. A prechemotherapy assessment that included sociodemographics, tumor/treatment variables, and geriatric assessment variables was performed. Association between these factors and the development of grade 3–5 toxicity was examined by using logistic regression. Results A total of 551 patients were accrued. Chemotherapy doses (odds ratio [OR] 1.834; 95% confidence interval [CI] 1.237–2.719) and creatinine clearance (OR 0.989; 95% CI 0.981–0.997) were the only factors independently associated with toxicity. Only 19% of patients who received reduced doses of chemotherapy and had a creatinine clearance ≥40 mL/minute had grade 3–4 toxicity, compared with 38% of those who received standard doses or had a creatinine clearance <40 mL/minute (p < .0001). However, no satisfactory multivariate model was obtained using different selection approaches. Conclusion Chemotherapy doses and renal function were identified as the major risk factors for developing severe toxicity in the older patient. These factors should be considered when planning to initiate a new chemotherapy regimen and should also lead to a closer follow-up in these patients.