Diabetes mellitus is a chronic disease characterised by hyperglycaemia resulting from deffects in insulin secretion, peripheral insulin resistance, or both. It is estimated that about 400,000 people in Croatia have diabetes. There are four basic types of diabetes: type 1 diabetes mellitus, type 2 diabetes mellitus, gestational diabetes and other specific types of diabetes. Treatment of diabetes includes keeping close watch over your blood sugar levels, with a combination of diet, exercise and, if necessary, medications. Complications of diabetes can be divided into two major categories: acute and chronic. Acute complications include hypoglycaemia, diabetic ketoacidosis and hyperglycaemic hyperosmolar state. Chronic complications (retinopathy, nephropathy, neuropathy) occur due to the damaging of blood vessels. Acute complications require prompt diagnosis and treatment. Diabetic ketoacidosis and hyperglycaemic hyperosmolar state are characterised by insulinopaenia and hyperglycaemia. Clinically, they differ only by the degree of dehydration and the severity of metabolic acidosis. The management of both states is based on the treatment of precipitating causes, fluid resuscitation and correction of hyperglycaemia, acidosis and electrolyte abnormalities. Hypoglycaemia is a frequent side effect of treating diabetes with medications. Lactic acidosis associated with metformin treatment is a rare but important side effect.