Diabetes mellitus (DM) is a lifelong progressive disease with high morbidity and mortality worldwide. Whereas cardiovascular complications are well-known for DM, increasing evidence indicates that diabetics are predisposed to cancer. Understanding of molecular pathomechanisms of cancer in DM is of great importance. Dysregulation of glucose/insulin homeostasis leads to increased production of Reactive Oxygen/Nitrogen Species (ROS/RNS) and consequent damage to chromosomal/mitochondrial DNA, a frequent finding in DM. Long-term accumulation of modified/damaged DNA is well-acknowledged as triggering cancer. DNA-repair is a highly energy consuming process provoking increased mitochondrial activity. Particularly dangerous is a provoked activity of damaged mitochondria leading to a "vicious circle" lowering energy supply and potentiating ROS/RNS production. Mitochondrial dysfunction may be implicated in pathomechanisms of diabetes-related cancer. High risk for infectious disorders and induced viral proto-oncogenic activity may further contribute to cancer provocation. Much attention should be focused on preventive measures in diabetic healthcare, in order to restrict severe diabetes-related complications.