Type 2 diabetes moderately increases predisposition for manifestation of tumor disease. Both drugs stimulating insulin secretion (insulin secretagogues) and insulin injection therapy also moderately increases risk of tumor manifestation (OR approx 1.3). According to some reports pyoglitazon therapy could be of increased risk of bladder cancer. On the other hand, hundreds of study on isolated cells, experimental animal models and retrospective studies in patients have shown preventive effect of metformin therapy on manifestation tumors of pancreas, breast, colorectum, liver, endometrium and ovary. More over, the prognosis of diabetic cancer patients on metformin therapy seems be better, than in diabetics without metformin treatment. These data are promising for future use of metformin for prevention and therapy of some malignant tumors.