Background and objectives: Weight loss and malnutrition are common among cancer patients, these two factors greatly affecting survival and quality of life during treatment. Since cancer is becoming increasingly common in the world and in order to provide better treatment measures, it is important to identify and prevent side effects. The present study has been conducted in 2010 on a sample of cancer patients in the oncology center of Shahid Beheshti University of Medical Sciences to determine the prevalence rates of malnutrition and the factors affecting it. Methods: The PG-SGA standard questionnaire was administered to 416 cancer patients to evaluate their nutrition status and determine the frequency of each malnutrition stage. Correlations and ANOVA tests were used to analyze the relationship between factors and weight loss and how they might affect the development of malnutrition. Results: The prevalence of malnutrition among the patients was 53.1% out of which 29.1% had moderate and 24% had severe malnutrition. The most common factors inducing nutritional symptoms were depression and anorexia. Some 35 % of the patients had over 5% weight loss in the last mouth. The average PG-SGA score was 10.1 with 49 being the highest. 46.1 percent of the patients scored over 9 (requiring critical nutrient intervention). Malnutrition has a high correlation with weight loss, activity limitations, nutritional symptoms, and cancer stage, but low correlation with treatment and pathologic type. Conclusion: Malnutrition has a high prevalence in Iranian cancer patients and has a close relationship with mortality, morbidity and treatment-related problems and also quality of life. Therefore, periodical assessment by PG-SGA to detect malnutrition in patients should be made so that appropriate nutritional interventions can be provided.