BACKGROUND Lung cancer is the leading cause of cancer-related mortality worldwide. Malnutrition is a common problem among patients with cancer, affecting up to 85% of patients with certain cancers and represents a risk factor for poor prognosis. aim: evaluate nutritional status in patients with lung cancer before and during treatment using nutritional risk index. METHODS it's a prospective study conducted in pneumology IV department in Abderahman Mami hospital, from January to May 2011. 30 male patients with a lung cancer were included. Nutritional status was assessed before and during treatment based on anthropometric measures, biological markers and nutritional risk index (NRI). RESULTS Mean age of patients was 58 ± 12 years, ranging from 19 to 82 years. 29 patients had non small cell lung cancer and one patient had small cell cancer. Malnutrition was noted in 14 patients (47%) before treatment according to the NRI. It was noted in 23 patients (77%) after three cycles of chemotherapy with severe malnutrition in 8 patients. Relationship between body mass index (BMI) and the NRI was linear, but NRI tends to evaluate more objectively risk of malnutrition in patients with lung cancer. CONCLUSIONS Nutritional assessment in patient with lung cancer should be performed systematically, early and repeatedly. Several markers can be used such as BMI and NRI. Nutritional support will reduce morbidity and improve quality of life in patients with lung cancer.