Multiple sclerosis (MS) is an inflammatory demyelinating disease of the CNS caused by the interplay of genetic and environmental factors. In the last years, it has been suggested that an abnormal venous drainage due to stenosis or malformation of the internal jugular and/or azygous veins may play a major pathogenetic role in MS. This abnormality called chronic cerebro-spinal venous insufficiency (CCSVI) could result in increased permeability of blood brain barrier, local iron deposition and secondary multifocal inflammation. In the present paper, literature data in favour and against this hypothesis are reported. A great variability of CCSVI has been found in both MS patients (ranging from 0 to 100%) and in control subjects (from 0 to 23%). This large variability is explained by methodological aspects, problems in assessing CCSVI, and differences among clinical series. It is urgent to perform appropriate epidemiological studies to define the possible relationship between CCSVI and MS.