The epidemiological studies have proven an increased incidence of cancer in patients with diabetes and the ne-gative effect of diabetes on their prognosis. The relation to type 2 diabetes was found in hepatic, pancreatic and endometrial malignancies, in colorectal, breast and bladder cancers. Diabetes as well as malignancies are multifactorial diseases with a number of common risk factors: age, gender, race, overweight and obesity, diet, physical activity. A higher incidence of malignancies in diabetic patients is not only a result of sharing these risk factors, as diabetes itself predisposes to carcinoma development. Possible mechanisms producing this effect include hyperinsulinemia, hyperglycemia, oxidative stress, chronic inflammation, obesity related factors and effect of diabetic complications. The development of malignancies in diabetics is also affected by the diabetes treatment. Exogenous insulin and its analogues and stimulators of insulin secretion are linked to an increased risk of cancer while insulin sensitizers to its decrease. Special attention is given to metformin which, apart from the indirect effect by influencing the levels of insulin and glycemia, has a direct anticancerogenic effect. In the clinical practice, it is ne-cessary to give attention to the screening of diabetes related tumours, consistent glycemic control and choice of appropriate diabetes treatment in patients with a high cancer risk.Key words: clinical practice - diabetes mellitus - effects of diabetes treatment - malignancies.