Background: The present study aimed to determine the extent of malnutrition in preoperative colorectal cancer patients. Malnutrition has been shown to affect post-operative outcome, so it would be beneficial to identify those who are malnourished or who are at risk of becoming so preoperatively. We examine whether weight loss is related to the length of stay or changes in fat free mass. Methods: Patients were enrolled consecutively from outpatients 2-4 weeks prior to surgery. Assessments included body mass index, percentage weight loss, dynamometry, Malnutrition Universal Screening Tool, Subjective Global Assessment and bioelectrical impedance. Cancer staging and hospital length of stay were recorded. Results: One hundred and thirty-two patients were eligible and 87 enrolled. Sixty-seven patients were weight losing and 20% had lost >10% of their usual body weight. Handgrip strength was lower in malnourished patients compared to those who had not lost weight (mean 19.4 and 27.3 kg, respectively, P = 0.013). Mean (SD) fat free mass in patients with a weight loss >10% was 39.7 (13.5) kg and, in those with <10% weight loss, was 51.9 (12.0) kg (P = 0.001). This difference was not demonstrated for fat. Conclusions: Over half of these patients had lost weight prior to surgery and one in five were malnourished. Body composition measurements demonstrated that malnourished patients had significantly less fat free mass compared to patients who were not clinically malnourished. Nutritional screening would be beneficial in this group preoperatively to identify weight-losing patients at an early stage in the care pathway when they initially enter the secondary care system. © 2010 The Authors. Journal compilation © 2010 The British Dietetic Association Ltd.