Chronic cerebro-spinal venous insufficiency (CCSVI) has been proposed as a “congenital malformation” implicated in the pathogenesis of multiple sclerosis (MS). However, numerous studies failed to confirm its presence in MS patients. This paper presents the rationale, design, and methodology adopted in the CoSMo study, conducted with the aim of verifying whether or not CCSVI is linked to MS. The primary endpoint of the CoSMo study is to compare the prevalence of CCSVI in patients with MS versus patients affected by other neurodegenerative diseases (OND) and healthy volunteers. CoSMo is a multicenter, blinded, prevalence study recruiting 2,000 adult subjects, involving 43 MS centers across Italy. Assessment of the presence or absence of CCSVI is performed by color-coded duplex (CCD) sonography and two out of the five criteria according to Zamboni are necessary for the diagnosis of CCSVI. Local CCD examination carried out by a certified sonologist and the central image readings performed by experts in the field are blinded. An advanced protocol is also described in this paper. The application of a rigorous methodological design will definitively confirm whether an association exists between CCSVI and MS. Should an association be observed, this study also further examines the link between CCSVI and the severity of MS. The addition of subgroups without MS and OND also provides information on whether CCSVI is specific to MS only. Results from the CoSMo study will play a crucial role in the possible studies concerning the potential treatment of CCSVI in MS.