Patients with or without preexisting diabetes undergoing treatment for cancer may be at risk for malglycemic events. Malglycemia, particularly hyperglycemia and diabetes in patients with cancer, may lead to adverse outcomes. Prevention, prompt recognition, and early intervention to regulate malglycemia can optimize the effects of cancer treatment, minimize the harmful consequences, and improve quality of life for patients with cancer. The development of evidence-based standards of care and protocols are needed to guide clinical practice when caring for patients with cancer.