INTRODUCTION: Older and frail patients with cancer are at high risk of physical and functional decline during chemotherapy. Exercise interventions can often counteract chemotherapy related toxicity and may help patients to improve or retain physical function and quality of life. Studies evaluating feasibility and the effect of exercise in older patients are lacking. The aim of this study was to investigate the feasibility and effect of an exercise intervention in older frail patients during chemotherapy for colorectal cancer (CRC). MATERIALS AND METHODS: This is a secondary analysis from the GERICO study investigating the effect of geriatric interventions in frail patients ≥70 years receiving chemotherapy for CRC. All patients in the present analysis were patients randomized to geriatric interventions and who were found physically frail (low handgrip strength or slow 10 m gait speed) and therefore offered referral to the exercise program for twelve weeks. We evaluated reasons for dropping out and feasibility of an individually tailored exercise program twice a week for twelve weeks. Each 60 min session comprised warm-up followed by progressive resistance training and cool-down followed by an oral protein supplement. Baseline characteristics and the effect of exercise for patients with high and low adherence (attendance of <50% of exercise sessions) were compared. RESULTS: Of 71 patients in the intervention group, 47 (66%) were found physically frail and were offered referral to the exercise program. Seven patients were referred to municipal physiotherapy before study start. In the remaining population (N = 40) 19 had exercise adherence >50% and 21 had no or low exercise adherence. Baseline characteristics were similar between patients with high and low/no adherence, except for sex (68% and 33% were men in high and low/ no adherence group, respectively). Patients with >50% attendance had significant improvements in physical tests after twelve weeks of exercise. DISCUSSION: Low adherence to the exercise program was seen due to lack of energy and/or treatment related adverse events. Patients with high adherence benefitted from exercise during chemotherapy but did not differ from patients with low adherence at baseline. Consequently, exercise should be offered to all older frail patients receiving chemotherapy for CRC.