Diabetes mellitus is a frequent comorbidity of cancer patients especially in the elderly. Recent data show that diabetes may negatively impact both cancer risks and outcomes of treatment. It is important to identify patients at risk for complications that arise from cancer treatment in the setting of pre-existing diabetes. Additionally, underlying hyperglycemia or hidden diabetes in a patient undergoing cancer treatment such as chemotherapy including steroid administration and total parenteral nutrition should be identified and managed. Strategies for monitoring and managing hyperglycemia in the elderly during the course of cancer treatment will be reviewed. The role of interdisciplinary care is crucial to supporting patients and their families as they manage through the challenges of facing two life-threatening diseases.