IMPORTANCE: The Cancer Aging Research Group (CARG) toxicity score is used to assess toxicity risk in geriatric patients receiving chemotherapy. OBJECTIVE: The primary aim was to validate the CARG score in geriatric patients treated with curative intent chemotherapy in predicting grade 3-5 toxicities. DESIGN: This was a longitudinal prospective observational study. SETTING: Tata Memorial Hospital, Mumbai, India, a tertiary cancer care referral centre. PARTICIPANTS: Patients, aged ≥65 years, with gastrointestinal, breast or gynaecological stage I-III cancers being planned for curative intent chemotherapy. A total of 270 patients were required for accrual in the study. EXPOSURES: Total risk score ranged from 0 (lowest toxicity risk) to 19 (highest toxicity risk). MAIN OUTCOMES AND MEASURES: The primary endpoint of the study was to evaluate whether the CARG risk score predicted for grade 3-5 toxicities. RESULTS: The study cohort of 270 patients had a mean age of 69 (65-83) years, with the most common cancers being gastrointestinal (79%). Fifty-two per cent of patients had atleast one grade 3-5 toxicity. The risk of toxicity was increased with an increasing risk score (42% low risk, 51% medium risk and 79% high risk; p<0.001). There was no association between either Eastern Cooperative Oncology Group (ECOG) performance status (p=0.69) or age-adjusted Charlson Comorbidity Index (p=0.79) risk categories and grade 3-5 chemotherapy toxicities. CONCLUSIONS AND RELEVANCE: This study validates the CARG risk score in predicting for grade 3-5 toxicities in geriatric oncology patients receiving curative intent chemotherapy and can be considered as the standard of care before planning chemotherapy in every elderly patient. TRIAL REGISTRATION NUMBER: CTRI/2016/10/007357; Results.