Background: Objective data for decision making about treatment options in elderly patients with cancer are lacking. The purpose of this study is to analyze potential predictive factors for the development of grade 3 or greater toxicity in elderly cancer patients on chemotherapy treatment Methods: 131 cancer patients ≥ 70 years were enrolled between January 2012 and December 2014. Five cases could not be analyzed because they are still under treatment ( ≤ 6 cycles). This study is a prospective cohort study approved by ethics committee of the participant hospitals. A comprehensive geriatric assessment (CGA) was completed before the treatment was decided and started. Geriatric data (Short portable mental status questionary-Pfeiffer-, body mass index (BMI), short physical performance battery –SPPB-, ADL, IADL, HDAS score, Charlson, VES-13, Hurria score and the MOS social scale) were collected. Cancer characteristics, social, demographic and laboratory data were also reviewed. Results: Median age of the patients was 77 years old (70-92), ECOG PS 0/1/2 were seen in 30%, 61% and 9% respectively. ADL and IADL showed impairment in 17% and 49% of the patients respectively, and VES-13 ≥ 7 in 14%. Forty-five (36%) patients developed grade 3-5 toxicity, with 1 % as grade 5 toxicity. Forty-one (32%) had a dose-intensity reduction. Univariate analysis found a higher risk grade 3-4 toxicity in patients with elevated GGT, Alkaline phosphatase, as well as low creatinine clearance, SPPB score and IMC and cancer type. In the multivariate analysis the significant predictive factors for grade 3-5 toxicity were cancer type (genitourinary or ginecologic) (OR, 8.274; 95% CI, [2.02 to 33.8]) and SPPB score (OR, 1.33; 95% CI, [1.03 to 1.73]). Conclusions: Genitourinary and ginecologic cancers and a poor performance in the short physical performance battery, were predictive of severe toxicity in elderly patients treated with chemotherapy.