BACKGROUND: Functional assessment of patients with cancer can be challenging and is often undertaken by the clinician with minimal direct input from the patient. We developed and aimed to validate the Elderly Functional Index (ELFI), a composite measure of self-reported functioning in older patients with cancer. METHODS: In this multicentre, prospective validation study, we validated ELFI in adult patients attending five oncology practices in Australia. ELFI is a 12-item composite measure of self-reported functioning derived from functional scales of the European Organisation for Research and Treatment of Cancer (EORTC) Quality of Life Questionnaire (QLQ): physical, role, and social functioning, and mobility. For evaluation of validity and internal consistency, participants self-completed ELFI, cognitive functioning and emotional functioning scales of the EORTC QLQ Core-30, Eastern Cooperative Oncology Group Performance Status (ECOG-PS), instrumental activities of daily living (IADL), and Clinical Frailty Scale (CFS) at baseline. For evaluation of test-retest reliability, participants opted in to repeat ELFI, cognitive functioning scale, emotional functioning scale, and ECOG-PS 1 week later, as well as completing the Global Rating of Change. Internal consistency reliability was assessed using Cronbach's α and test-retest reliability was assessed using intraclass correlation (ICC). We assessed ELFI for convergent and discriminant validity (Spearman's r), known-groups validity (ANOVA), and structural validity (exploratory factor analysis). FINDINGS: Between May 6 and Dec 15, 2017, 877 participants with cancer returned a total of 869 baseline questionnaires and 482 retest questionnaires. 621 (71%) participants (192 [31%] aged ≥70 years) were included in evaluations of validity and internal consistency and 278 (32%) participants (106 [38%] aged ≥70 years) in evaluations of test-retest reliability. ELFI demonstrated excellent internal consistency reliability (Cronbach's α=0·93 for all participants; p<0·0001) and test-retest reliability (overall ICC 0·90, 95% CI 0·87-0·92; p<0·0001). Hypotheses regarding convergent and discriminant validity were confirmed, with all item-scale correlations exceeding 0·40 except for one on the physical functioning scale. ELFI was better than its component scales and other function measures at differentiating between groups with different function and frailty scores (known-groups validity). Exploratory factor analysis provided empirical support to the structural validity of ELFI. Strong correlation was observed between ELFI and its component scales (r ranging from 0·67 to 0·79), ECOG-PS (-0·79), IADL (0·69), and CFS (-0·73). INTERPRETATION: ELFI is a validated and simple person-reported multidimensional measure of functional status, which captures broad dimensions of functioning. ELFI has enhanced statistical efficiency relative to its components, reducing the sample size required to detect a given effect. ELFI could be used as a clinical trial endpoint to assess functional domains of health-related quality of life. FUNDING: National Health and Medical Research Council, Monash University, Eastern Health.