BACKGROUND: The decision whether to treat older adults with advanced cancer with standard therapy (ST) or reduced therapy (RT) is complicated by heterogeneity in aging. We assessed the potential utility of the chemotherapy toxicity risk score (CTRS) [J Clin Oncol 2011;29:3457-3465] for treatment decisions in older adults. MATERIALS AND METHODS: This was a prospective observational study of patients aged ≥65 years receiving first-line chemotherapy for advanced cancer for which combination chemotherapy is the standard of care. Patients were categorized as high risk (CTRS ≥10), for whom RT (dose-reduced combination or single-agent chemotherapy) is deemed appropriate, or nonhigh risk (CTRS <10), for whom ST is deemed appropriate for toxicity. The primary objective was to estimate the agreement in chemotherapy choice (ST vs. RT) between the treating physician and the CTRS using a κ statistic. RESULTS: Fifty-eight patients (median age, 71 years) were enrolled. Thirty-eight patients received ST (21 had CTRS <10, and 17 had CTRS ≥10), and 20 patients received RT (12 had CTRS ≥10, and 8 had CTRS <10), with minimal agreement in chemotherapy choice (κ = 0.14; 95% CI, -0.10 to 0.38). Grade 3-4 toxicity and hospitalization occurred in 60% and 27% of 55 patients with follow-up data, respectively. Among patients receiving ST, patients with CTRS ≥10 had a higher incidence of toxicity (88% vs. 40%, p = .006) and hospitalization (50% vs. 15%, p = .03) than those with CTRS <10. CONCLUSION: Older patients with cancer with a high CTRS who receive combination chemotherapy have an exceedingly high rate of severe toxicity and hospitalization. IMPLICATIONS FOR PRACTICE: The potential utility of the chemotherapy toxicity risk score (CTRS) in old adults with advanced solid tumors receiving first-line chemotherapy was assessed. Little agreement was found between chemotherapy treatment decisions based on the clinical impression versus what was recommended based on the CTRS. Among patients treated with standard-dose combination chemotherapy, patients with CTRS ≥10 had a very high incidence of grade 3-4 toxicities and hospitalization, which was significantly greater than that of patients with a low CTRS (<10). These findings suggest that the addition of CTRS to the clinical impression has a potential to improve treatment decisions.