The Comprehensive Geriatric Assessment (CGA) represents the future of the geriatric oncology to reduce toxicities and treatment-related hospitalization in the elderly. Most patients receiving docetaxel for metastatic castration-resistant prostate cancer are in their seventies or older. We explored the efficacy of the CGA in predicting chemotherapy feasibility and response to docetaxel in a cohort of 24 patients aged at least 70. This was an observational, prospective study involving 24 patients who were 70 years of age or older and about to start chemotherapy with docetaxel for metastatic castration-resistant prostate cancer; we performed a CGA including five domains and divided our patients into 'healthy' and 'frail'; the relations between general condition and (i) early chemotherapy discontinuation and (ii) response to docetaxel were explored. We found a statistically significant relationship between frailty assessed by CGA and early docetaxel discontinuation; we also found an association between frailty and response to chemotherapy, but this did not reach statistical significance. A geriatric assessment before starting chemotherapy may help clinicians to recognize frail patients, and hence to reduce toxicities and early treatment discontinuation. Further analyses are required to simplify the CGA tools and to facilitate its incorporation into routine clinical practice.