The field of oncogeriatrics considers the comprehensive geriatric assessment (CGA) as the main tool for distinguishing between patients who are frail and those who are not frail. The aim of our study was to determine the role of the CGA in predicting the risk of frailty in elderly patients. This prospective study was conducted at the Cancer in the Elderly Unit of the Medical Oncology Department at the Virgen de la Luz General Hospital in Cuenca, Spain. Demographic data and information about the CGA were collected. Using a bivariate logistic regression analysis, these factors were analysed and the factors that are associated with the risk of frailty were determined, as measured by the Barber questionnaire (BQ). We included 262 patients in the study with a mean age of 79 years (range 7093 years). Seventy-four percent of the patients (n=194) had a risk of frailty as measured by the BQ. In the bivariate analysis, only age (odds ratio [OR] 1.064, 95 % confidence interval [CI] 1.0001.133, p=0.051), being divorced, widowed or single (OR 0.450, 95 % CI 0.2160.937, p=0.033) and being dependent in instrumental activities of daily living (IADL) (OR 3.003, 95 % CI 1.1817.638, p=0.021) were associated with a higher risk of frailty. The risk of being frail in an elderly patient with cancer is higher in patients dependent in IADL and in patients who are not married. Age is another risk factor for frailty.