context: the body mass index (bmi) is the most common marker used on diagnoses of the nutritional status. the great advantage of this index is the easy way to measure, the low cost, the good correlation with the fat mass and the association to morbidity and mortality. objective: to compare the bmi differences according to the who, opas and lipschitz classification. methods: a prospective study on 352 patients with esophageal, gastric or colorectal cancer was done. the bmi was calculated and analyzed by the classification of who, lipschitz and opas. results: the mean age was 62.1 ± 12.4 years and 59% of them had more than 59 years. the bmi had not difference between the genders in patients <59 years (p = 0.75), but over 59 years the bmi was higher in women (p<0.01). the percentage of undernourished was 7%, 18% and 21% (p<0.01) by who, lipschitz and opas, respectively. the overweight/obesity was also different among the various classifications (p<0.01). conclusions: most of the patients with gastrointestinal cancer had more than 65 years. a different cut off must be used for this patients, because undernourished patients may be wrongly considered well nourished.