Cancer patients receiving chemotherapy have a high risk of malnutrition secondary to the disease and treatment, and 40–80 % of cancer patients suffer from different degrees of malnutrition, depending on tumour subtype, location, staging and treatment strategy. Malnutrition in cancer patients affects the patient's overall condition, and it increases the number of complications, the adverse effects of chemotherapy and reduces the quality of life. The aim of the present study was to evaluate weight-loss prevalence depending on the tumour site and the gastrointestinal (GI) symptoms of oncology patients receiving chemotherapy. We included 191 cancer patients receiving chemotherapy. Files of all patients were reviewed to identify symptoms that might potentially influence weight loss. The nutritional status of all patients was also determined. The cancer sites in the patients were as follows: breast (31·9 %); non-colorectal GI (18·3 %); colorectal (10·4 %); lung (5·8 %); haematological (13·1 %); others (20·5 %). Of these patients, 58 % experienced some degree of weight loss, and its prevalence was higher among the non-colorectal GI and lung cancer patients. Common symptoms included nausea (59·6 %), anorexia (46 %) and constipation (31·9 %). A higher proportion of patients with ≥ 5 % weight loss experienced anorexia, nausea and vomiting (OR 9·5, 2·15 and 6·1, respectively). In conclusion, these results indicate that GI symptoms can influence weight loss in cancer patients, and they should be included in early nutritional evaluations.