Evidence exists that the geriatric intervention guided by Comprehensive Geriatric Assessment (CGA) has positive effects on a number of important health outcomes in frail older patients. Although a number of observational studies, editorials, special articles and clinical reports, suggest that CGA should be used to guide the assessment and clinical decision-making in older cancer patients, there is limited support to this view in the literature. Older patients that are diagnosed with cancer are usually healthier and less problematic than persons of the same age who are randomly sampled from the general population. In these persons, the cancer dominates the clinical picture and, therefore, instruments especially tuned for the frail elderly may provide little information. The concept of the frailty syndrome, characterized by high susceptibility, low functional reserve and unstable homeostasis, has recently received a lot of attention by the geriatric community. A CGA approach, which also evaluates elements of the frailty syndrome, may be of great interest for those oncologists who want to identify older patients likely to develop severe toxicity and severe side effects in response to aggressive treatment. Improvements in the definition of the frailty syndrome may profit from the clinical experience of oncologists.