Aim To determine whether carrying out the Comprehensive Geriatric Assessment before operations would be useful for predicting complications, particularly postoperative delirium (POD), in old‐old patients. Methods A total of 517 patients aged 75 years and older, who underwent radical surgery for gastrointestinal cancer at Osaka University Hospital, were recruited for this observational study. The Comprehensive Geriatric Assessment components and assessment of performance status were carried out before surgery, and a record of postoperative complications including POD was made prospectively until discharge from hospital. The following morphological and clinical measurements were also obtained from the medical records: age, sex, disease type, previous history, comorbid lifestyle‐related diseases, POD, postoperative complications, operative method, duration of operation, hemorrhage volume, blood transfusion volume, method of anesthesia, body mass index and blood tests. Results POD appeared in 24.0% of the 517 patients who underwent surgery. Barthel Index, Mini‐Mental State Examination, instrumental activities of daily living and Geriatric Depression Scale results were associated with the incidence of POD, and the Barthel Index, Mini‐Mental State Examination and Instrumental Activities of Daily Living results were extracted as independent factors associated with the development of POD after adjusting for traditional risk factors for postoperative complications and performance status. Conclusions The Comprehensive Geriatric Assessment before gastrointestinal surgery can be a useful tool for predicting the development of POD in old‐old patients. Geriatr Gerontol Int 2016; 16: 1036–1042.