Patients with diabetes mellitus are at a higher risk to undergo surgical intervention compared with the non-diabetic population, and additionally, they have an increased perioperative morbidity and mortality. Insulin deficiency and insulin resistance are aggravated by surgery and anaesthesia. The consequences of hyperglycemia are glycosuria, volume depletion from osmotic diuresis, impairment of wound healing and leucocyte function and exacerbation of ischemic brain damage. Depending on the extent of hypoinsulinemia, lipolysis and ketogenesis are enhanced which may result in metabolic acidosis with subsequent electrolyte disturbances. Protein catabolism is increased because of increased breakdown and decreased synthesis. Insulin administration reverts or overcomes most of these disturbances. The preoperative assessment includes the diagnoses of the long-term complications to judge the intraoperative risks. Long-acting insulins, such as ultralente of animal origin should be stopped preoperatively and substituted by protamine and lente insulins. In type-2-diabetic patients, long-acting sulfonylurea drugs such as chlorpropamide should be stopped and substituted by short-acting agents. Metformin must always be stopped. Type-2-diabetic patients with marked hyperglycemia under oral treatment should be switched to insulin before operation. The insulin requirements in diabetic patients during surgery vary from 0.25-0.40 U per gram glucose in normal weight patients, 0.4-0.8 U per gram glucose in case of obesity, liver disease, steroid therapy or sepsis, to 0.8-1.2 U per gram glucose in patients undergoing cardiopulmonary bypass surgery. Therefore, the appropriate dose has to be determined individually. The regimen nowadays preferred by most authors is based on variable rate insulin infusion.(ABSTRACT TRUNCATED AT 250 WORDS)