The contribution of subcutaneous or intra-abdominal fat (SCF, IAF) accumulation to postoperative mortality or morbidity after elective gastric or colorectal cancer surgery was studied. One hundred thirty-nine patients who underwent gastric or colorectal cancer surgery were investigated. Data were collected on the following preoperative comorbid conditions: lung dysfunction, cardiovascular dysfunction, liver dysfunction, or diabetes mellitus (DM). The IAF and SCF areas were determined at the umbilical level by preoperative CT scanning. We investigated whether IAF or SCF was independently associated with postoperative mortality or morbidity. The mortality rate was 0.7 per cent (1/139). Nineteen (14%) of 139 patients had medical complications (pneumonitis or arrhythmia) and 27 (19%) had surgery-related complications (anastomotic leakage, intra-abdominal collections, or abdominal wound infection) postoperatively. Age (> or =70 years), lung dysfunction, cardiovascular dysfunction, and IAF were significantly associated with medical complications, and surgical site (gastric/colorectal), DM, body mass index, IAF, and SCF were significant for surgery-related complications by univariate analysis. Multilogistic regression analysis showed that age, cardiovascular dysfunction, and IAF were significant for medical complications, and surgical site, DM, and SCF were independently related with surgery-related complications. Body fat accumulation is independently associated with postoperative morbidity after gastric or colorectal surgery, and assaying it may be useful for predicting postoperative complications.