Dextran is often used to enhance the successful outcome of microvascular transplantations. So far, some 60 cases of dextran-induced acute renal failure have been reported. Among them, only one has been reported as an adverse reaction to antithrombotic prophylaxis during microvascular surgery. To stress that dextran prophylaxis can be a serious threat to microsurgical patients, the authors report on two more such patients and discuss how to prevent and treat this condition. Hypovolemic patients of older age are at risk, as are patients with cardiovascular diseases, renal artery stenosis, and preexisting renal insufficiency. Dextran should be discontinued promptly in patients with decreased urinary output or with high urine specific gravity. Dextran-induced acute renal failure may be severe and it may last several days. Plasmapheresis is the therapy of choice and, usually, diuresis and renal function resume shortly after such treatment.