At present the main attention in the care of diabetic patients is mainly focused on the prevention and treatment of chronic complications of diabetes mellitus.Acute complications are not at all the past. Causes of their origin, clinical presentation and methods of treatment marked significant changes. Acute complications of diabetes mellitus can be divided into: metabolic decompensations and complications of antidiabetic treatment. To metabolic decompensations belong acute hyperglycemic states (diabetic ketoacidosis and hyperglycemic hyperosmolar nonketotic state). Diabetic ketoacidosis is acute metabolic emergency mainly of typ 1 diabetes mellitus. It is characterised by: hyperglycemia, acidosis and ketosis. Hyperglycemic hyperosmolar nonketotic state is acute complication mainly of type 2 diabetes mellitus. The ketoacidosis is not present, what is explained by higher intraportal insulin level, preventing mobilisation of free fatty acids. Lactic acidosis is the most serious complication of metformin therapy mainly in patients having risk factors. For this reason it is necessary to observe contraindications of metformin therapy. Hypoglycemias are the most frequent acute complications mainly in diabetics treated by insulin or sulphonylurea derivatives.