Although malnutrition is known to be frequent in cancer patients, it has not been described in a selected population of patients with gastrointestinal malignancies under chemotherapy only. Physician judgment about malnutrition and risk factors for malnutrition were also evaluated. All consecutive in- and outpatients of 11 centers were prospectively enrolled in a cross-sectional 14-day period study and classified according to the French health recommendations [Haute Autorité de Santé (HAS)]. Among 313 patients enrolled in 11 centers (mean age = 63 yr; range = 21-93; 67% male) mainly with colorectal (58%), pancreatic (15%), gastric (11%), and hepatobiliary (10%) primary tumors, the prevalence of malnutrition was 52%. Moderate and severe malnutrition was present in 27% and 25% of cases, respectively. Physicians considered it in 36% and 6% of cases, respectively, thereby misclassifying 134 patients (43%). The agreement between the HAS definition and the physicians' judgment was very low (κ = 0.30). Most of the patients who were identified as severely malnourished received no nutritional support. Performance status and pancreatic and gastric cancers were independently associated with malnutrition. Malnutrition levels are high, around 50%, unequally distributed according to the primitive tumor. It is still underestimated by physicians. Weight loss remains a clinically relevant, simple, and reliable marker of malnutrition.