Background: Chronic cerebrospinal venous insufficiency (CCSVI) has been proposed as a possible cause of multiple sclerosis (MS). Objectives: The CoSMo study evaluated the association between CCSVI and MS. Methods: The primary end-point of this multicentric, case-control study was to compare the prevalence of CCSVI between patients with MS, patients with other neurodegenerative diseases (ONDs) and healthy controls (HCs). Color-coded duplex sonography was performed by a sonologist and the images were sent to one of three central sonologists for a second reading. Agreement between local and central sonologists or, in case of disagreement, the predominant judgment among the three central readers, was required for a diagnosis of CCSVI. All readings, data collection and analysis were blinded. Results: The study involved 35 MS centers across Italy and included 1874 subjects aged 18–55. 1767 (94%) were evaluable: 1165 MS patients, 226 patients with ONDs and 376 HCs. CCSVI prevalence was 3.26%, 3.10% and 2.13% for the MS, OND and HC groups, respectively. No significant difference in CCSVI prevalence was found amongst the three cohorts (MS versus HC, OR = 1.55, 95%CI = 0.72–3.36, p = 0.30; OND versus HC, OR = 1.47, 95%CI = 0.53–4.11, p = 0.46; MS versus OND, OR = 1.05, 95%CI = 0.47–2.39, p = 0.99). High negative and low positive agreement was found between the local and centralized readers. Conclusions: CCSVI is not associated with MS.