In 100 patients who underwent major cranial or spinal operations, the incidence of lower extremity deep vein thrombosis was 29%. Of importance was the presence of known risk factors, particularly leg weakness and a long operation. The subject of deep vein thrombosis and its complications in neurosurgical disorders is reviewed and its prophylaxis is discussed. The administration of low dose heparin based on an epidemiological analysis of the risks involved would seem to be an effective method of prophylaxis.