Emerging evidence on body composition suggests that sarcopenic obesity (obesity with depleted muscle mass) might be predictive of morbidity and mortality in non-malignant disease and also of toxicity to chemotherapy. We aimed to assess the prevalence and clinical implications of sarcopenic obesity in patients with cancer. Between Jan 13, 2004, and Jan 19, 2007, 2115 patients with solid tumours of the respiratory or gastrointestinal tract from a cancer treatment centre serving northern Alberta, Canada, were identified. Available lumbar CT images of the obese patients were analysed for total skeletal muscle cross-sectional area; these values were also used to estimate total body fat-free mass (FFM). Of the 2115 patients initially identified, 325 (15%) were classified as obese (body-mass index [BMI] > or =30). Of these obese patients, 250 had CT images that met the criteria for analysis. The remaining 75 patients were recorded as without assessable scans. Obese patients had a wide range of muscle mass. Sex-specific cut-offs that defined a significant association between low muscle mass with mortality were ascertained by optimum stratification analysis: 38 (15%) of 250 patients who had assessable CT images that met the criteria for analysis were below these cut-offs and were classified as having sarcopenia. Sarcopenic obesity was associated with poorer functional status compared with obese patients who did not have sarcopenia (p=0.009), and was an independent predictor of survival (hazard ratio [HR] 4.2 [95% CI 2.4-7.2], p<0.0001). Estimated FFM showed a poor association with body-surface area (r(2)=0.37). Assuming that FFM represents the volume of distribution of many cytotoxic chemotherapy drugs, we estimated that individual variation in FFM could account for up to three-times variation in effective volume of distribution for chemotherapy administered per unit body-surface area, in this population. This study provides evidence of the great variability of body composition in patients with cancer and links body composition, especially sarcopenic obesity, to clinical implications such as functional status, survival, and potentially, chemotherapy toxicity.