Background: Predicting major adverse events such as functional decline during chemotherapy is an important issue in geriatric oncology. Recent studies have shown that different parts of comprehensive geriatric assessment (CGA) (Geriatric Depression Scale and Instrumental Activities of Daily Living) were predictive of functional decline. However CGA is time-consuming and has to be performed by trained geriatricians who are not in sufficient number to face the increasing demand of elderly cancer patients. Thus screening tools, such as G8-score validated on 1435 patients, have been developed to identify patients who may benefit from CGA. In addition to its value as a screening tool, the value of the G8 as a predictive tool of survival has been suggested and we here investigate whether it can be used to predict functional decline during chemotherapy. Methods: We tested on a cohort of 364 patients over 70 year-old treated with first-line chemotherapy the value of clinical (G8, age, sex, performance status –PS-, disease localization, extension) and biological factors (platelets, creatinine clearance, albumin, CRP, neutrophils) in predicting early functional decline, defined as a decrease of 0.5 points on the Activities of Daily Living (ADL) scale between the beginning of chemotherapy and the second cycle. We performed a multivariate analysis using logistic regression model. Results: Of the 364 patients, 312 were assessable for functional status and 68 experienced functional decline. On univariate analysis, PS 2 to 4, low platelets, metastatic disease and abnormal G8 ( ≤ 14) score were associated with functional decline. On multivariate analysis, abnormal G8 score (OR, 3.56; 95% CI 1.22; 10.34; p = 0.02) and PS 2 to 4 (OR, 2.0; 95% CI 1.1; 3.6; p = 0.04) were significantly associated with increased likelihood of early functional decline. Conclusions: G8 has been developed as a screening tool to predict abnormal geriatric assessment. We have shown here that it could also be used together with performance status to select elderly patients more likely to develop functional decline during chemotherapy. These data reinforce the routine use of G8 in all elderly patients treated for cancer.