BMI is a simple and widely recorded variable that may capture obesity or cachexia in cancer patients. How BMI is associated to health-related quality of life (HR-QoL) in such patients is poorly investigated. High BMI may be associated to obesity, an increased burden of comorbidity, reduced physical activity and, in some settings, to more aggressive oncological disease. On the other hand, low BMI may reflect enhanced weight loss, cachectic syndrome, higher tumor burden and adverse prognostic features which all deteriorate quality of life. The aim of the present study is to evaluate the association of BMI and HR-QoL as measured by the EORTC-QLQ-C30 questionnaire in several cancer settings (such as localized vs metastatic or distinct primary tumors).