Both diabetes and cancer are common diseases that have a great impact on health worldwide. Many cancer patients have diabetes as a comorbidity. Increased frequency of concurrent cancer and diabetes has been described in epidemiological studies for many types of cancer such as liver, endometrial, colorectal and breast cancer. The association between these two diseases may be due to hyperglycemia and common risk factors. Patients diagnosed with cancer who have preexisting diabetes are at an increased risk for long-term all-cause mortality compared with those without diabetes. Both diabetes and anticancer chemotherapy cause similar complications, namely cardiovascular problems, peripheral neuropathy, renal impairment and infections. Because of these common complications, close monitoring of side effects and prevention is necessary in this group of patients.